We investigated the action of capsazepine, an antagonist of the actions of capsaicin on sensory neurones, on the contractile responses evoked by capsaicin or by electrical field stimulation (EFS) in guinea-pig bronchi. Capsazepine (10(-5) M) selectively inhibited responses to capsaicin, producing a significant change in EC50 values but not the Hill coefficient (nH), suggesting that capsazepine acts as a competitive antagonist (apparent pKB = 5.12) whereas ruthenium red is a non-competitive antagonist. Capsazepine and ruthenium red were without effect on EFS-induced responses.