Calcium-dependent release of [3H]GABA was elicited by electrical stimulation in slices of rat and mouse cerebral cortex or by potassium stimulation in the mouse brain-stem. The stimulation-evoked release of [3H]GABA was inhibited by yohimbine in a concentration-dependent manner. High concentrations of other alpha-adrenoceptor antagonists such as phentolamine, RS 21361, idazoxan, rauwolscine and corynanthine also inhibited [3H]GABA release. This effect was not observed with pseudoyohimbine or prazosin. [3H]GABA release was not affected by exposure to the alpha-adrenoceptor agonists clonidine, M7, noradrenaline or methoxamine. In addition, clonidine did not antagonize the yohimbine-induced inhibition of [3H]GABA release. The inhibitory effect of yohimbine did not result from an interaction with endogenously released noradrenaline since the inhibition was still observed in reserpine-pretreated animals. It is concluded that yohimbine and other alpha 2-adrenoceptor antagonists inhibit the stimulation-evoked release of [3H]GABA through a mechanism which appears to be independent of the blockade of alpha 2-adrenoceptors and which does not involve an interaction with endogenous noradrenaline. The present results indicate that yohimbine exerts non-specific actions on the release of [3H]GABA and that similar effects can be observed with other alpha-adrenoceptor blocking agents in high concentrations. Consequently, when studying the effects of yohimbine and other alpha 2-adrenoceptor antagonists on noradrenergic neurotransmission, the possibility of non-specific effects should be taken into consideration, particularly in the high concentrations range.