The behavioral effects of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME), when perinatally (2 d prenatal-14 d postnatal) co-administered with extremely low frequency magnetic fields, were examined in weanling and adult rats. Litters of rat pups and their dams were exposed continuously to biphasic pulsed fields presented once every 2 s. The magnetic fields were amplitude modulated in successively increasing and decreasing steps (each 30 min) between 0 and 1.8 microT or between 0 to 13 nanoT (reference field) during 4-h periods (6 periods per day). These two treatments were subdivided into dams that received tap water and dams that received 1.0 g/L L-NAME in tap water. The behavioral sequelae to these treatments for 242 progeny from 41 litters were followed from weaning (1 wk after termination of treatment) into adulthood. Compared to exposures to water and nanoT magnetic fields, perinatal exposures to the microT magnetic fields or to L-NAME in the maternal water supply were associated with increased activity levels when the rats were tested as weanling, but decreased activity levels when the rats were tested as adults. However, the activity of rats that received the combination of L-NAME and microT magnetic fields did not differ significantly from the activity of the rats that had received water and the nanoT fields. Long-term (adulthood) effects of these perinatal treatments on associative learning, as inferred by learned fear to contextual stimuli, were not evident. These results indicate that L-NAME and this particular pattern of magnetic field antagonized one another when co-administered during the perinatal period.