Hendricks would have turned 65 today.
Mike Flanagan, a friend of Mr. Hendricks', confirmed the death but didn't want to comment until he had notified other members of the organization.
Mr. Hendricks died at Baltimore-Washington Medical Center in Glen Burnie, hospital spokeswoman Allison Eatough told the Associated Press. She did not know the time of death.
Acting Lt. Will Bethea of the Anne Arundel County Fire Department told the news service that the department received a report at 8:17 p.m. of an unconscious man at the BWI Marriott hotel in Linthicum. A department ambulance took the man to the medical center, he said.
Washington Nationals manager Frank Robinson, a Hall of Fame outfielder who played alongside Mr. Hendricks and later managed teams that Hendricks coached, told The Sun last night: "You can't do justice to the man. You can say nice things about him, but you can't do him justice. He was such an outstanding individual and outstanding baseball person. He had a kind word for everybody and a smile for everybody."
Reached at his home last night, Orioles owner Peter G. Angelos declined to comment, saying he needed to get his thoughts together first.
Mr. Hendricks, known for his quick wit and smile, dressed as Santa Claus at the Orioles' Christmas party Monday and delivered gifts to children from economically disadvantaged areas in Baltimore.
Before putting on the Santa suit, he talked about how much he enjoyed the annual Christmas party where he interacted with kids and served them lunch.
Hendricks, who had a minor stroke in mid-April, also said he felt great health-wise, but joked that at "age 65, you always wake up with different aches and pains."
Mr. Hendricks suffered the stroke during the final game of a three-game series against the Tampa Bay Devil Rays in mid-April. He missed 19 games while recovering, and when he returned to the Orioles' clubhouse, he said he had gained a new perspective after the health scare.
In early October, Mr. Hendricks said that he had gotten a clean bill of health from his doctor.
He was named bullpen coach after the 1977 season, during Earl Weaver's first stint as Orioles manager, and was reassigned in October as club officials cited concerns about his health.
Orioles manager Sam Perlozzo said at the time that it was "probably the hardest decision" he has ever had to make.
"I consider Elrod one of my best friends," Mr. Perlozzo said in October. "Elrod is someone we love dearly, and we want to make sure he is going to be OK."
Mr. Perlozzo couldn't be reached for comment late last night. Mr. Hendricks' role with the organization for next season was undecided, but the club was hoping he would continue as team ambassador. Mr. Hendricks routinely spent the hours before the first pitch meeting fans, posing for pictures and singing autographs.
He said Monday that he was not looking forward to being out of uniform during games.
"I know no other way," he said Monday. "This spring, when I got sick, I didn't like watching the games on television, so I can't imagine this would be any better. At least right now, I am thinking of not watching or listening to the games. We'll see what happens. But it is going to be difficult."
Last season was Mr. Hendricks' club-record 37th in an Orioles uniform and 28th as bullpen coach. Since 1968, when Hendricks joined the organization as a catcher, he had been with the Orioles for all but 1 1/2 of 37 seasons.
"He touched an awful lot of people both on and off the field," Robinson said. "He was a tremendous ambassador for the game of baseball and for the Orioles. He cannot be replaced. No one was like him. No one is going to be like him after this.
"When he came over to the Orioles, everybody told me he was the Babe Ruth of Mexico. I don't know about that, but nobody could throw a fastball by him. He was a tremendous asset when we won our world championships. He was a good friend. I'm stunned. I send my condolences to Merle and his family."
Hall of Fame pitcher Jim Palmer, a former teammate said: "We all are stunned. Elrod's Elrod. All the great things the franchise has accomplished, he has been a part of. ... What's not like about Elrod? He always had a smile on his face.
"Unfortunately, that's part of growing older. You get robbed of your good friends and compatriots. Elrod is the kind of guy who made it all worthwhile. Not only because of what he did for the Orioles, but also in the community. He was a great goodwill ambassador. He was kind of a rock."
Rick Dempsey, a current Orioles coach and former teammate of Mr. Hendricks', said: "I'm pretty shocked. We thought that Elrod was out of the woods and that everything was going to be OK. I talked to people who saw him this morning and said he looked great. With Pat Kelly leaving because of a heart attack and now Elrod, what next? This organization has just been through hell. It's just been one thing after another.
"He's been kind of a mentor for all of us. He's been there and seen so much and done so much. Even when I came back to the Orioles, I looked to Elrod for advice about everything with the ballclub. He was such a good man. This is a sad day for the Orioles. It really is."
Mr. Hendricks, a catcher who was born in the Virgin Islands, spent 12 seasons in the major leagues, including 10 1/2 with the Orioles. He played in more than 100 games in each of the Orioles' pennant-winning seasons from 1969 through 1971.
The Orioles drafted him from the California Angels' Triple-A affiliate in 1967 after Hendricks had played for Mr. Weaver in Puerto Rico.
Mr. Hendricks broke into professional baseball in 1959 and made his major league debut with the Orioles in 1968. He played in 711 games - including 658 with the Orioles - before retiring in 1979.
Mr. Hendricks was inducted into the Orioles Hall of Fame on Aug. 26, 2001.
"All my rewards come from being in this uniform," he said the day he was inducted. "This is a bonus and something I never expected. The rewards I've gotten from this game far outweigh what I've put into it. For me, this is icing on the cake."
Mr. Hendricks filled in as manager twice in 1988, when Robinson was away from the time. Mr. Hendricks interviewed in 1994 for the managerial job after Johnny Oates was fired. The position went to Phil Regan.
Mr. Hendricks is survived by his wife of 35 years, Merle; their sons, Ryan, 33, and Ian Christopher, 29; and four children from Mr. Hendricks' previous marriage, Abegail, 43; Elrod Jr., 41; Elroy, 40; and Berecia, 38.
Sun reporter Nicole Fuller contributed to this article.