Diazepam is one of the benzodiazepines, a group of drugs that depresses the central nervous system. It also inhibits the contractility of smooth muscles in the periphery, but the mechanism of this inhibitory action has not been clarified. Our study was undertaken to investigate the effect of diazepam on the contractility of the detrusor muscle. Detrusor muscle strips isolated from rat urinary bladder were examined by isometric myography. Diazepam, as well as baclofen, a gamma-aminobutyric acid (GABA)B receptor agonist, reduced the electric field stimulation-induced contractions; delta-aminovaleric acid, a GABAB receptor antagonist, completely antagonized the inhibitory effect of baclofen, but not the inhibitory action of diazepam. Diazepam reduced the basal tone of detrusor muscle concentration dependently, and this inhibitory action was not affected by tetrodotoxin. Diazepam suppressed the contractile responses to bethanechol, adenosine triphosphate and potassium chloride. Diazepam diminished the calcium-induced recovery of tension in calcium-free PSS. A23187, a calcium ionophore, partially recovered the basal tone which had been reduced by diazepam in normal physiologic salt solution (PSS). The loss of tension in calcium free PSS containing diazepam could not be recovered by addition of A23187. On the other hand, the loss of tension in calcium-free PSS containing 3,4,5-trimethoxybenzoic acid 8(diethylamino)octyl ester (TMB-8), an inhibitor of intracellular calcium release, was considerably recovered by addition of A23187. Based on these results, it is suggested that diazepam inhibits the contractility of detrusor muscle acting directly on the smooth muscle cell, which is unrelated to the activation of GABA receptors. Its inhibitory action appears to be mediated through interference with the influx of extracellular calcium.