The effect of biscoclaurine (bisbenzylisoquinoline) alkaloids on phospholipase A₂ and C activation in signal transduction system of rabbit platelet was studied. Isotetrandrine, cepharanthine and berbamine inhibited the aggregation induced by collagen but not by other stimuli such as thrombin and arachidonic acid, while tetrandrine equally inhibited the aggregation by any of these agonists. All these four alkaloids suppressed arachidonic acid liberation in response to collagen or thrombin, but not diacylglycerol formation and increase in cytoplasmic Ca²⁺ concentration in response to thrombin or arachidonic acid. In saponin-permeabilized platelets, they also suppressed arachidonic acid liberation induced by an addition of both GTPγS and Ca²⁺, whereas the liberation induced by an addition of Ca²⁺ alone was not prevented by them. These data suggest that isotetrandine, cepharanthine and berbamine have a rather specific potency to suppress the phospholipase A₂ activation by a mechanism other than direct inhibition of the enzyme or interference with the ligand-receptor interaction. They seem, at least in part, to exert the effect on the GTP-binding protein-phospholipase A₂ complex in the platelet signal transduction system. In contrast, tetrandine appears to inhibit a step following an increase in cytosolic free Ca²⁺ concentration in the agonist-induced signal transduction system, in addition to suppressing the phospholipase A₂ activation.