Motility recordings in muscle strips from rat urinary bladder were performed and the effect of the opiate agonist loperamide on motor activity was studied. Loperamide induced a concentration-dependent (10(-7)-10(-3) M) inhibition of the contractile response of the detrusor strip of the same order of magnitude after activation of intramural nerves, stimulation of cholinergic receptors with acetylcholine and after direct depolarisation of the cell with potassium. Pretreatment with the opiate-antagonist naloxone (10(-5) M) did not antagonise the inhibitory action of loperamide on bladder motility regardless of the type of activation. Naloxone per se, however, facilitated the nerve-mediated motor response. The inhibitory action of loperamide on the potassium-induced contraction could partly be counteracted by elevation of the calcium concentration in the medium. It is suggested that the demonstrated inhibitory effect of loperamide on bladder motility is a calcium-dependent direct smooth muscle action, without any significant opiate-receptor-mediated action in the present in vitro preparation.