A new study with laboratory rats finds that a mentally stimulating environment can reverse the learning deficits caused by lead poisoning, suggesting a possible remedy to the long-term harm done to thousands of children poisoned every year.

Researchers at the Johns Hopkins University Bloomberg School of Public Health found that lead-poisoned rats raised in an "enriched" environment - one with multiple levels that included toys, tunnels, running wheels and other rats - did as well as healthy rats in finding a hidden platform in a pool of water.

The same rats did considerably better than lead-poisoned rats raised in isolation in regular laboratory-sized cages - nearly half of whom could not find the platform even by the last day of training.

"This is the first evidence to show that the cognitive or learning impairments produced by exposure to lead are reversible," said Tomas R. Guilarte, a professor of environmental health sciences at Hopkins and lead author of the study, which appeared yesterday in the online edition of the Annals of Neurology.

Guilarte said researchers have not determined how long-lasting the effects of the stimulating environment may be. Nor do they know whether putting rats in such an environment long after exposure to lead would produce the same benefits.

But Guilarte said the results of the study could have implications for the millions of children suffering the effects of lead poisoning worldwide, mainly from ingesting flakes or dust from old lead-based paint.

"It's basically a demonstration that some of these [early childhood development] programs that are out there, like Head Start, could be very beneficial for children that have been exposed to lead," he said.

Lead impedes proper brain and nerve development and can cause hearing loss, reduction of motor control and balance and learning disabilities in young children. It has also been associated with a broad range of behavioral problems, including hyperactivity and increased aggression.

The state Department of the Environment reported that 2,841 children tested last year in Maryland - including 2,027 in Baltimore - had elevated blood-lead levels. Nationally, about 890,000 children ages 1 through 5 have elevated lead levels, according to the U.S. Centers for Disease Control and Prevention.

Dr. Herbert L. Needleman, a professor of psychiatry and pediatrics at the University of Pittsburgh and a leading expert in lead poisoning, called the study "intriguing" but said he hadn't yet read it. His research has shown that deficits caused by lead exposure appear to be permanent.

"The outlook for kids with elevated lead burdens we thought was pretty bad, and that's been borne out," he said.

Needleman cautioned that the Hopkins study involved only rats and that children could respond differently.

"It's a different species with a different set of arrangements, so I have to look at it to see," he said.

Needleman said studies dating to the 1960s have shown the difference between rats raised in "enriched" and "impoverished" environments. The rats that had toys were smarter than the ones that didn't.

Guilarte said he and his colleagues were surprised by how much differently the rats in the stimulating environments behaved, even from the first day.

"We never expected it to be this clear-cut," he said.