Recent laboratory tests found so much mercury in tuna sushi from 20 Manhattan stores and restaurants that at most of them, a regular diet of six pieces a week would exceed the levels considered acceptable by the Environmental Protection Agency.

Sushi from 5 of the 20 places had mercury levels so high that the Food and Drug Administration could take legal action to remove the fish from the market. The sushi was bought by The New York Times in October.

¡°No one should eat a meal of tuna with mercury levels like those found in the restaurant samples more than about once every three weeks," said Dr. Michael Gochfeld, professor of environmental and occupational medicine at the Robert Wood Johnson Medical School in Piscataway, N.J.

Dr. Gochfeld analyzed the sushi for The Times with Dr. Joanna Burger, professor of life sciences at Rutgers University. He is a former chairman of the New Jersey Mercury Task Force and also treats patients with mercury poisoning.

The owner of a restaurant whose tuna sushi had particularly high mercury concentrations said he was shocked by the findings. ¡°I'm startled by this,¡± said the owner, Drew Nieporent, a managing partner of Nobu Next Door. ¡°Anything that might endanger any customer of ours, we¡¯d be inclined to take off the menu immediately and get to the bottom of it.¡±

Although the samples were gathered in New York City, experts believe similar results would be observed elsewhere.

¡°Mercury levels in bluefin are likely to be very high regardless of location,¡± said Tim Fitzgerald, a marine scientist for Environmental Defense, an advocacy group that works to protect the environment and improve human health.

Most of the restaurants in the survey said the tuna The Times had sampled was bluefin.

In 2004 the Food and Drug Administration joined with the Environmental Protection Agency to warn women who might become pregnant and children to limit their consumption of certain varieties of canned tuna because the mercury it contained might damage the developing nervous system. Fresh tuna was not included in the advisory. Most of the tuna sushi in the Times samples contained far more mercury than is typically found in canned tuna.