Experiments examining seizure sensitivity were conducted on adult male offspring exposed to diazepam at 1.0 or 2.5 mg/kg per day in utero over gestational days 14-20. Threshold dosages to facial clonus, myoclonic jerk, clonic seizure, and extensor tonus were determined via i.v. infusion of bicuculline, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), picrotoxin, pentylenetetrazol, caffeine and strychnine. Relative to uninjected and vehicle-exposed adult male offspring, prenatal diazepam administration reduced the threshold for bicuculline- and DMCM-induced facial clonus and myoclonic jerk by 40-50%. The threshold dosages to facial clonus, myoclonic jerk and clonic seizure from picrotoxin infusion were similarly reduced in animals exposed to diazepam in utero. In contrast, seizure thresholds to pentylenetetrazol, caffeine and strychnine were not affected by early developmental exposure to diazepam. In parallel biochemical studies, an increased sensitivity to the antagonistic effects of bicuculline methiodide on gamma-aminobutyrate (GABA)-stimulated chloride influx was observed in cortical synaptoneurosomes from adult male progeny of diazepam-treated dams. The results are interpreted to reflect a long-lasting alteration in the function of the GABA/benzodiazepine receptor complex by prenatal diazepam exposure that is manifest at the behavioral and neurochemical level in a pharmacologic specific manner.