1 Recent studies have demonstrated growing evidence for a primary action of the benzodiazepines on gabaminergic neurones which induces a facilitation of gamma-aminobutyric acid (GABA)-mediated neurotransmission. As enhancement of GABA release has been suggested to account for their activation of GABA mechanisms, the effect of diazepam and clobazam, and of several other psychotropic drugs, on stimulated GABA release have been studied. 2 Using rat brain cortex slices saturated with [3H]-GABA, the electrically stimulated overflow of GABA is reduced in a concentration-dependent manner in the presence of both diazepam and clobazam. 3 The benzodiazepine-induced reduction in GABA overflow during electrical stimulation is antagonized by the GABA receptor blocker bicuculline, whereas bicuculline alone at 10(-6) M concentration does not change the overflow. 4 Among some other centrally active drugs tested, hexobarbitone and the 'second messenger; cyclic GMP also induce a significant but less marked reduction in GABA release. 5 A schematic model of a central gabaminergic synapse is proposed, which may explain the benzodiazepine effects on stimulated GABA release by suggesting an inhibitory feedback control of transmitter release mediated by presynaptic GABA receptors ('autoreceptors').