We examined the effects of gestational and lactational exposure to polychlorinated biphenyl (PCB) 153 (2,4,5,2',4',5'-hexaCB) on the magnitude of long-term potentiation (LTP) observed in the CA1 region of hippocampal brain slices prepared from rats at 30 days of age. We compared these actions to those observed when PCB 153 is dissolved in normal Krebs-Ringer solution and perfused on slices from control rats of the same age. In vivo exposure was at three dose levels (1. 25, 5, and 20 mg/kg/day) from gestational day 3 through weaning at postnatal day 21. Although responses to low-frequency stimulation of the Schaffer collateral pathway in exposed animals were not different from controls, significantly reduced LTP was induced after tetanic stimulation, even at the lowest dose studied. We observed a comparable depression of LTP when control slices were perfused with Krebs-Ringer that had been equilibrated with PCB 153 in a generator column. Neither in vivo nor in vitro exposure significantly altered the input-output curves obtained before tetanic stimulation, but both suppressed the increase in response observed in controls after tetanic stimulation. Because LTP is thought to be correlated with learning ability, these observations may provide at least a partial mechanism to explain the reduction of intelligence quotient observed in humans exposed to PCBs early in development.