Two hundred thirty-six children from two established cohorts at risk for exposure to polychlorinated biphenyls (PCBs) and related contaminants were evaluated at age 4 years. Prenatal exposure (assessed by cord serum PCB level) was associated with lower weight, an effect consistent with reports of growth retardation in laboratory rats and in children exposed at high levels in Taiwan and at general population levels in Japan. The highest exposed children weighed 1.8 kg less on the average than the least exposed. Contemporary body burden (assessed by 4-year serum PCB level) was associated with reduced activity based on composite ratings provided by the child's mother and two independent examiners. This effect, attributable to lactation exposure, was strongest among the offspring of women with above average milk PCB levels who breast fed for at least 1 year. While the weight deficit is consistent with previous data linking developmental effects of low-dose human PCB exposures specifically to the prenatal period, activity is the first domain found to be affected by lactation at contemporary levels of exposure.