Prior to 1989, American infants generally received three vaccinations (polio, measles-mumps-rubella, and diphtheria-tetanus-pertussis). In the early 1990s, public health officials dramatically increased the number of Thimerosal-containing vaccinations without considering the cumulative impact of the mercury load on developing brains.

In a 1991 memo, Dr. Maurice Hilleman, one of the fathers of Merck's vaccination programs, warned his bosses that 6-month-old children administered the shots on schedule would suffer mercury exposures 87 times the government safety standards. He recommended that Thimerosal be discontinued and complained that the US Food and Drug Administration, which has a notoriously close relationship with the pharmaceutical industry, could not be counted on to take appropriate action as its European counterparts had. Merck ignored Hilleman's warning, and for eight years government officials added seven more shots for children containing Thimerosal.

Mercury is a known brain poison, and autism rates began rising dramatically in children who were administered the new vaccine regimens. A decade ago the American Academy of Pediatrics estimated the autism rate among American children to be 1 in 2,500. Today, the CDC places the rate at 1 in 166, or 1 in 80 boys. Additionally, one in six children is now diagnosed with a related neurological disorder.

In 2000, the CDC met with pharmaceutical companies and the FDA in secret to review its findings linking Thimerosal with the dramatic rise in neurological illnesses. According to transcripts, participants were alarmed about the undeniable links between the Thimerosal and widespread brain damage in children. Dr. Bill Weil told the group, ''You can play with [the results] all you want. They are statistically significant." Dr. Richard Johnston admitted he feared his grandchild getting a Thimerosal-containing vaccine. But the group was most concerned with keeping the findings secret. ''Consider this embargoed information," said Dr. Roger Bernier, a senior director at the National Immunization Program, at the meeting's close. The CDC now says it has ''lost" the data that supported the crucial study and has persistently defied congressional requests and federal law requiring it to open up the federal Vaccine Safety Database to scientists and the public.

Numerous animal, DNA, epidemiological, and other studies point to Thimerosal as a culprit in America's epidemic of neurological disorders. Autistic children have been shown to have higher mercury loads than nonautistics, and there have been reports of significant improvements in some brain-injured children by removing mercury from their brains. Most of the symptoms of autism are similar to the symptoms of mercury poisoning. Scientists have been able to induce autism-like symptoms in mice by exposing them to Thimerosal. A recent study by an FDA scientist, Dr. Jill James, found that many autistic children are genetically deficient in their capacity to produce glutathione, an antioxidant generated in the brain that helps remove mercury from the body.

Government health agencies who green-lighted Thimerosal have turned a blind eye to the hundreds of studies linking Thimerosal to a wide range of neurological disorders and joined the pharmaceutical industry to gin up a series of flawed European studies to exonerate Thimerosal. Those studies examined children exposed to a tiny fraction of the Thimerosal given to American kids and took advantage of the autism spike that resulted from deceptive data-gathering in Scandinavia to argue that autism rates are unrelated to Thimerosal use.

Drug makers wary of liability reduced Thimerosal in most children's vaccines in recent years, but the preservative remains in flu shots, tetanus boosters, and over-the-counter drugs. Mercury-laced vaccine stocks were given to American children until the end of 2003.

Government officials who continue to champion Thimerosal should recognize that this is not just a theoretical exercise in bureaucratic face-saving. Their wrong-headed defense of Thimerosal safety in the face of overwhelming science is discouraging testing of promising treatments which may be effective. They are depriving vulnerable populations from being identified to avoid Thimerosal. They also cannot escape responsibility for their failure to warn international health agencies and governments who, based upon American assurances, are now injecting the developing world's children with this brain-killing chemical.

Robert Kennedy Jr. is senior attorney for the Natural Resources Defense Council.