The Environmental Protection Agency plans to conduct a new study about the potential health risks of atrazine, a widely used weedkiller that recent research suggests may be more dangerous to humans than previously thought.

Atrazine — a herbicide often used on corn fields, golf courses and even lawns — has become one of the most common contaminants in American drinking water.

For years, the E.P.A. has decided against acting on calls to ban the chemical from environmental activists and some scientists who argued that runoff was polluting ecosystems and harming animals.

More recently, new studies have suggested that atrazine in drinking water is associated with birth defects, low birth weights and reproductive problems among humans, even at concentrations that meet current federal standards.

The E.P.A. is expected to announce on Wednesday that it will conduct a new evaluation of the pesticide to assess any possible links between atrazine and cancer, as well as other health problems, such as premature births. The E.P.A. may determine that new restrictions are necessary.

The decision by E.P.A.’s administrator, Lisa P. Jackson, who took over the agency in January, is a significant departure from the policies of the E.P.A. under President George W. Bush.

For years, agency officials said that atrazine in drinking water posed almost no risk to humans or the environment. As recently as this summer, E.P.A. staff members argued that current regulations were adequate.

“We’re going to use our scientific resources in a new and more aggressive way regarding atrazine,” said Stephen A. Owens, who was recently confirmed as E.P.A. assistant administrator for prevention, pesticides and toxic substances.

“There are new scientific findings that deserve attention, and we’re going to engage our scientific panels in actively reviewing the work of this office under previous administrations,” he added. “We have a question: Did the decisions made in previous administrations use all the available science?”

A representative of atrazine’s largest manufacturer, the Swiss company Syngenta, said that she had not been fully briefed on the E.P.A.’s announcement. However, the spokeswoman, Sherry Ford, said, “we expect a positive outcome for atrazine at the end of this process.”

Ms. Ford added that the company “stands behind the safety of atrazine, which has undergone extensive testing. We are a science-based company, and we expect the E.P.A. to make sound decisions based on science, no matter which administration is currently in power.”

Observers say the E.P.A.’s announcement signals a significant shift.

“This is a dramatic change,” said Linda Birnbaum, director of the National Institute of Environmental Health Sciences, part of the Department of Health and Human Services. “There is growing evidence that atrazine could be a hazard to human health. This is a strong signal that the world is changing for some of the most widely used chemicals.”

Atrazine has become a lightning rod in disputes over how the E.P.A. has used scientific findings to regulate chemicals and toxins.

The agency was sued in 2003 by an environmental advocacy organization, the Natural Resources Defense Council, amid claims that regulators had ignored studies showing that atrazine was dangerous to some animals.

In August, The New York Times reported on recent epidemiological studies that suggested small amounts of atrazine in drinking water, including levels considered safe by federal standards, might be associated with birth defects — including skull and facial malformations and misshapen limbs — as well as premature births and low birth weights in newborns.

E.P.A. officials said those studies, as well as recent papers reviewing numerous studies that showed that atrazine interferes with the development and hormone systems of some animals, played a role in their decision to re-evaluate the chemical.

A Times analysis of E.P.A. records also found that in some American towns, atrazine concentrations in drinking water had spiked sharply, sometimes for as long as a month. Though the E.P.A. and Syngenta were aware of those spikes, they often did not promptly warn local water systems, and the reports produced by local regulators and distributed to residents often failed to reflect those higher concentrations. Interviews with local water officials indicated that many of them were unaware that atrazine concentrations sometimes jumped sharply in their communities.

But officials in other communities have grown concerned. Water systems in six states — Illinois, Indiana, Iowa, Kansas, Mississippi and Ohio — recently sued atrazine’s manufacturers to force them to pay for removing the chemical from drinking water.

The E.P.A. is expected to announce on Wednesday four meetings over the coming year of the agency’s independent scientific advisory panel that will focus on atrazine.