The levels of polychlorinated biphenyls, a toxic chemical that has been regulated since 1976, have declined sharply in the U.S. population, a new Environmental Protection Agency study reported yesterday.

The EPA said that the percentage of the population having more than three parts per million of PCBs in their fat tissue had dropped from 8 percent in 1977 to 1 percent in 1981. "This is a dramatic example of the improvements that environmental regulations are bringing us. It is heartening to see that the many regulations phasing out the uses of PCBs are being effective," said Don Clay, the EPA's acting assistant administrator for pesticides and toxic substances.

While the study found that high levels of PCBs were declining in the U.S. population, the number of Americans with at least some PCBs in their bodies was estimated to have increased. The study said that in 1972 85 percent of the population had detectible levels of PCBs, a figure that had climbed to 100 percent in 1981.

While PCBs can no longer be manufactured in the United States, the EPA estimates that 250 million pounds of PCBs are still in use or storage around the country, mostly in large electrical equipment. Some PCBs are also produced as a byproduct in manufacturing. The same study found that levels of DDT, a pesticide banned in 1972, were also decreasing, although 99 percent of the population had detectable amounts of DDT in their fatty tissue.