Mid Atlantic Medical Services Inc. (MAMSI), a Rockville health insurer, reported yesterday that its third-quarter earnings rose 49.8 percent, easily beating analysts' expectations.

The insurer had net income of $22.5 million, or 54 cents a diluted share, in the three months that ended Sept. 30., up from $15 million, or 38 cents a share, in the third quarter last year.

Analysts, on average, had expected earnings of 49 cents a share.

Commercial health maintenance organization membership was 16.6 percent higher than in the year-earlier quarter.

Membership growth was slower in its preferred-provider plans, but those offer less revenue and less profit per member than do the commercial HMOs, which account for half of MAMSI's membership, now almost 2 million.

The higher membership helped increase revenue to $595.5 million in the quarter, up 29.5 percent from $459.7 million in the third quarter last year.

MAMSI reported that average monthly premiums per member were up 11.9 percent. Medical costs rose at the same rate, but lower administrative costs (as a percentage of revenue) and the higher membership accounted for the increase in earnings.

MAMSI operates in Maryland, Virginia, North Carolina, Pennsylvania, West Virginia, Delaware and the District of Columbia. Its HMOs are M.D. IPA and Optimum Choice. Alliance is its preferred provider organization.

The earnings were reported after the close of the New York Stock Exchange, where MAMSI shares ended the day at $36.60, up $1.95.