The effects of capsaicin on the ability of platelets to aggregate in response to thrombin, platelet-activating factor or calcium ionophore (A23187) were examined. At concentrations previously shown to activate sensory afferent neurons, capsaicin markedly inhibited the responsiveness of platelets to the three agonists. The effects of capsaicin on platelet aggregation were reversible, and could be observed if capsaicin was added after platelets had begun to aggregate in response to the agonist. Capsaicin did not affect the shape change which occurs in response to the agonists, a process which is calcium-independent. These results demonstrate that capsaicin, at concentrations which are frequently used to 'selectively' activate sensory afferent neurons, is also capable of affecting the function of the platelet. Such non-specific effects of capsaicin must be considered when this substance is used as a pharmacological probe of sensory afferent nerve function.