The neuroactive steroid 5 beta-pregnan-3 alpha-ol-20-one (5 beta 3 alpha) modulates GABAA receptor function by potentiating postsynaptic GABA currents. While much is now known about the postsynaptic action of neurosteroids, far less is known about how they affect neurotransmission. We have investigated the synaptic actions of 5 beta 3 alpha in a simple vertebrate model, the embryo of the clawed toad, Xenopus laevis, in which a known GABAergic pathway, activated by the rostral cement gland, terminates swimming when the animal contacts an obstruction. Cement gland stimulation evokes bicuculline-sensitive inhibitory postsynaptic potentials (IPSPs) in motorneurones that terminate swimming and which are greatly enhanced by the presence of (1-5 microM) 5 beta 3 alpha. In the presence of TTX, depolarising inhibitory potentials are recorded with KCl-filled microelectrodes reflecting the spontaneous release of transmitter. The majority are glycinergic with durations of 20-80 ms and are blocked by strychnine while the remainder are GABAergic with durations of 90-200 ms and are abolished by bicuculline. We show here that, in the presence of 5 beta 3 alpha, the spontaneous GABA IPSPs lengthen dramatically in some cases to over 500 ms, but the glycine potentials are unaffected. The steroid has no other detectable postsynaptic effects in that the range of amplitudes of GABA potentials is unaffected and there is no change in the resting membrane potential. However, 5 beta 3 alpha also caused a marked increase in the rate of occurrence of spontaneous GABA potentials. This suggests a novel presynaptic site of action in which the steroid enhances the probability of vesicular GABA release from GABA terminals.