If confirmed by further experiments, the findings could help explain the decline in human male fertility over the past 50 years.

The study was carried out on laboratory rats that received high levels of vinclozolin, a fungicide widely used in vineyards, and methoxychlor, a pesticide used to replace DDT when it was banned more than 30 years ago. Scientists found that the male offspring of the exposed rats suffered a sharp decline in the quality and quantity of their sperm and that these traits continued to be passed on down the male line.

Yet the researchers believe that the chemicals did not mutate the genes of the rats -- a proven way of passing on damaging traits -- but instead may have altered the way the genes work.

New Way to Think About Disease

Michael Skinner of Washington State University, who led the research team, said nearly all the male rats born in each generation were affected by sperm damage or low sperm counts. He said that the findings, published today in the journal Science, suggest that toxins may play a role in heritable diseases that were previously thought to be caused solely by genetic mutations.

"It's a new way to think about disease. We believe this phenomenon will be widespread and be a major factor in understanding how disease develops," Dr. Skinner said.

The rats were exposed to much higher levels of the toxic chemicals than would be experienced even by agricultural workers handling the products on a daily basis. But the scientists believe that this does not rule out the possibility that a similar effect may result from exposure to low doses.

No Obvious Mutations in the DNA

Both of the chemicals are known to be toxic in high doses and each is considered capable of interfering with the functioning of reproductive hormones -- a feature of toxins known as endocrine disrupters.

The scientists exposed pregnant rats to the chemicals at the crucial moment in gestation when the sex of the offspring is determined. The result was that male offspring suffered a 20 percent decline in sperm counts, and sperm motility -- its ability to swim -- fell by up to 35 percent.

What was surprising was that these traits were also seen in 90 percent of the male offspring born to three more subsequent generations yet the scientists found no obvious mutations in the DNA of the animals.

One possibility is that the toxic substances altered the natural chemicals, called methyl groups, that normally surround the DNA molecule and these subtle changes were inherited by the male offspring.

"We are mostly describing a new phenomenon ... The hazards of environmental toxins are much more pronounced that we realized," Dr Skinner said.