We examined the effect of a kappa agonist, U-50488H, upon the bladder motility of anaesthetized rats. The frequency of distension-induced rhythmic bladder contractions was reduced by the intravenous (10 mg kg(-1)) or intrathecal (10-100 microg) administration of U-50488H. The effect of intravenous U-50488H was inhibited by pre-treatment with nor-binaltorphimine (10 mg kg(-1), s.c.). The inhibition of bladder contractions by intrathecal U-50488H (30 microg) was eliminated with the concomitant use of nor-binaltorphimine (10 mg kg(-1), s.c.), and diminished by reserpine (4 mg kg(-1), i.p.), yohimbine (10 microg, i.t.) or methysergide (20 microg, i.t.). The amplitude of bladder contractions induced by an electrical stimulation of the pontine micturition centre was not inhibited by intrathecal U-50488H (30 and 100 microg). These results suggested that a kappa agonist could inhibit micturition reflex as well as other opioids, and at least part of the inhibition was due to the diminishment of bladder sensation based on the activation of the descending monoaminergic systems through the spinal kappa-opioid receptors.