1. We have investigated the effect of nociceptin on the micturition reflex evoked by distension or topical application of capsaicin on the urinary bladder of urethane-anaesthetized rats. 2. Nociceptin produced a dose-dependent (3-100 nmol kg(-1) i.v.) transient suppression of the distension-evoked micturition reflex: its effect was not modified by guanethidine (68 micromol kg(-1) s.c.) nor by bilateral cervical vagotomy, alone or in combination, and by naloxone (1.2 micromol kg(-1) i.v.). 3. Nociceptin (100 nmol/kg i.v.) slightly (about 30%) inhibited the contractions of the rat bladder produced by pre- or postganglionic electrical stimulation of the pelvic nerve. 4. Nociceptin almost totally abolished the reflex component of the response to topical capsaicin (1 microg in 50 microl). 5. In the rat isolated bladder, submaximal contractions produced by electrical field stimulation were slightly reduced (25+/-4% inhibition) by 1 microM nociceptin. Nociceptin did not affect the contraction of the rat bladder induced by acetylcholine (10 microM) or ATP (1 mM). 6. These findings indicate that nociceptin exerts a naloxone-resistant suppression of the volume-evoked micturition reflex which involves inhibition of transmitter release from postganglionic bladder nerves. An inhibitory effect on bladder afferent nerves is also suggested.