The action of (+/-)-kavain on the veratridine, monensin and KCl-depolarization evoked increase in free cytosolic Ca2+ concentration ([Ca2+](i)), and its influence on the release of endogenous glutamate from rat cerebrocortical synaptosomes were investigated. [Ca2+](i) was fluorimetrically determined employing FURA as the Ca2+ sensitive fluorophore, and glutamate was detected by a continuous enzyme-linked fluorimetric assay. The incubation of synaptosomes in the presence of (+/-)-kavain up to a concentration of 500 mu mol/l affected neither basal [Ca2+](i) nor spontaneous release of glutamate, but dose-dependently reduced both veratridine-elevated [Ca2+](i) (IC50 = 63.2 mu mol/l) and glutamate-release (IC50 = 116.4 mu mol/l). The inhibition of these parameters, attained with 500 mu mol/l (+/-)-kavain, could be overcome by inducing an artificial Na+ influx, using monensin as a Na+ ionophore. An application of (+/-)-kavain after veratridine caused a decrease in veratridine-elevated [Ca2+](i), which was similar to the action of tetrodotoxin (TTX) with regard to time course, half-life of [Ca2+](i) decline and the final steady state level of [Ca2+](i). Concomitantly, veratridine-induced glutamate-release was blocked. The results indicate that specific inhibition of voltage-dependent Na+ channels is a primary target of (+/-)-kavain, thus preventing a [Na+](i) provoked increase in [Ca2+](i) and glutamate-release. However, pathways related to the elevation of [Ca2+](i) by [Na+](i) itself, and the processes involved in normalization of elevated [Ca2+](i) and glutamate-release downstream to enhanced [Ca2+](i), seems to be unaffected by (+/-)-kavain. Using KCl-depolarized synaptosomes, 400 mu mol/l (+/-)-kavain reduced, in analogy to Aga-GI toxin, KCl-evoked [Ca2+](i) and diminished the part of glutamate-exocytosis which is related to external Ca2+ to about 75% of control. At a concentration of 150 mu mol/l, which is above the IC50 value necessary to block voltage-dependent Na+ channels, (+/-)-kavain affected neither basal nor the KCl-induced increase in [Ca2+](i). These results might suggest that (+/-)-kavain at concentrations sufficient to block Na+ channels completely, moderately inhibits the non-inactivating Ca2+ channels located on mammalian presynaptic nerve endings.