SR 48968 (10(-6) to 10(-5) M) inhibited the cholinergic response of the isolated guinea-pig main bronchus to electrical field stimulation. Since this effect was reversed by naloxone 10(-5) M and since SR 48968 had no effect on the contractile response to exogenous acetylcholine, we conclude that SR 48968 acts at a prejunctional level and that opioid receptors are involved. This effect was observed at concentrations approximately 75,000 times higher than those needed for blockade of tachykinin NK2 receptors.