The involvement of gamma-aminobutyric acid (GABA) in the vagal-stimulated reflex inhibition of sympathetic nerve discharge (SND) was investigated in the cat. Computer summation was used to assess the sympathoinhibitory response to vagal afferent stimulation and the resultant changes seen with agents known to effect GABAergic neurotransmission. The GABA antagonists picrotoxin and bicuculline at 0.25 to 1.0 mg/kg i.v. attenuated the vagal sympathetic inhibitory response, but increased the 1:1 locking of SND to the arterial pulse. Conversely, the GABA agonist diazepam at 0.3 mg/kg i.v. potentiated the vagal sympathoinhibition, reduced total SND and diminished SND locking to the arterial pulse. Picrotoxin also blocked the vagal sympathoinhibitory response in midcollicular transected cats and when applied topically to the dorsal brain stem. Midcollicular transection forestalled the irregular SND spiking normally seen with picrotoxin in the intact cat, thus resulting in improved 3 cycle/sec SND periodicity. None of these drug-induced changes effected the SND shutoff response to vasopressor tests, however. These results suggest that GABA plays a role in the SND inhibitory response to vagal afferent stimulation in the brain stem, independent of the sinoarterial baroreceptors, and may also be involved in the entrainment of SND to the arterial pulse.