The effect of the centrally acting muscle relaxant, (4S,5R)-4-(2-methylpropyl)-3-[3-(perhydroazepin-1-yl)propyl]-5- phenyl-1,3-oxazolidin-2-one (NC-1200) on micturition contractions of the urinary bladder was tested in anesthetized rats with the intention of using it clinically to depress vesical micturition contractions. Rhythmic micturition contractions of the bladder were produced by expanding a balloon inserted in the bladder. Efferent nerve activity of the pelvic and hypogastric branches of the bladder was recorded. Both the vesical micturition contractions and the rhythmic burst discharges of the pelvic efferent nerve were inhibited by the i.v. administration of NC-1200 (1-10 mg/kg) in a dose-dependent manner. Hypogastric efferent nerve activity and ganglionic synaptic transmission from the pelvic preganglionic neurons to their postganglionic neurons innervating the bladder were not influenced significantly by the i.v. administration of NC-1200. No inhibitory effect on vesical smooth muscle was observed. From these results we concluded that NC-1200 inhibits the micturition contraction by inhibiting pelvic efferent nerve activity and that this inhibitory effect of NC-1200 originates mainly in the central nervous system.