Superfusion of crude synaptosomal fractions from the rat brain cortex was used to study in vitro the effect of phenibut (beta-phenyl-GABA), a tranquilizing agent, on spontaneous and K+-stimulated release of 3H-GABA. It was found that phenibut in concentrations of 50 and 100 microM enhanced spontaneous tritium efflux by 15.9 and 30.7%, respectively. The effect of exogenous GABA in a concentration of 100 microM was 5 times more remarkable. The K+-stimulated release of the transmitter significantly increased under the effect of phenibut. Bicuculline counteracted this enhancement while picrotoxin did not effect the K+-stimulated egress of 3H-GABA. It is assumed that the presynaptic component described plays a role in the realization of the tranquilizing action of the tranquilizing action of phenibut.