As previously reported by this laboratory, an endogenous factor capable of inhibiting the specific binding of the radiolabeled cannabinoid agonist [3H]CP-55940 to its receptor can be released from nerve terminals in response to an influx of Ca++ induced by an ionophore (Evans et al., 1992). In the present report, we provide evidence that the endogenous ligand for the cannabinoid receptor can be released in response to a depolarizing stimulus (75 mM K+) in the presence of extracellular Ca++. K(+)-evoked release was not observed in the absence of extra-cellular Ca++ and was reduced by the specific calcium channel blockers verapamil and omega-conotoxin. The efflux of cannabinoid receptor binding activity is greatest within 2 min of stimulation with the Ca++ ionophore A23187. Within this period of time, the cannabinoid receptor binding activity was enhanced by the presence of a cocktail of peptidase inhibitors. Examination of the contribution of individual inhibitors for enhancing high K(+)-released material revealed a selectivity for captopril and thiorphan. The specificity of the released factor for the cannabinoid receptor was corroborated by its ability to compete with the aminoalkylindole radioligand [3H]WIN-55212 for binding to this receptor. Fractions from a semi-purified sample of the effluent demonstrated binding to the cannabinoid receptor and behaved as agonists in that these fractions could inhibit adenylate cyclase activity in neuroblastoma membrane preparations.