[3H]Ketanserin was found to label (besides a low amount of 5-HT2 receptors) non-serotonergic binding sites on human platelet membranes. The latter binding was detected in the presence of excess of the 5-HT2 antagonist BW501, and was potently inhibited by tetrabenazine. [3H]Ketanserin revealed a KD value = 19 +/- 4 nM and a Bmax = 425 +/- 82 fmol/10(9) platelets for these binding sites. [3H]Ketanserin binding in the presence of BW501 was inhibited by the tetrabenazine derivative RO-4-1284 (IC50 = 1.4 nM), tetrabenazine (IC50 = 8.6 nM) and the ketanserin derivatives R 71,278 (IC50 = 6.3 nM), R 47,288 (IC50 = 17 nM) and R 71,428 (IC50 = 100 nM). Ketanserin revealed an IC50 = 32 nM. The drugs were found to trigger the release of 3H from [3H]5-HT-loaded human platelets in superfusion experiments in vitro. The amount of 3H released by the drugs correlated with their binding affinities for the non-serotonergic sites. The non-serotonergic [3H]ketanserin binding sites on human platelets and their possible role in triggering monoamine release corresponded to the properties of non-serotonergic ketanserin binding sites previously characterized in rat striatum. The possible role of the action of ketanserin on the non-serotonergic sites in the reported partial reduction by ketanserin of the monoamine content in cardiovascular tissues is discussed.