The effect of antibodies to GM1 ganglioside on release of neurotransmitters from rat brain slices was studied. Depolarization-induced (40 mM-KCl or veratrine) release of gamma-aminobutyric acid was markedly enhanced. Depolarization-induced release of norepinephrine was only slightly enhanced, whereas that of serotonin was unaffected. No effect on spontaneous release was observed for any of these three neurotransmitters. These results show that antibodies that can bind to synaptic membrane antigens may alter neurotransmitter release and that antibodies directed against GM1 ganglioside exhibit a measure of specificity in producing such an effect.