We studied the effects of baclofen on the bladder contraction induced by infusion of Tyrode's solution into the urinary bladder in anesthetized rats. Baclofen (5 mg/kg, i.v.) completely inhibited bladder contraction and abolished the efferent discharges recorded from the left pelvic nerve, causing the bladder pressure to rise until solution leaked from the penis. The inhibitory effect of baclofen (5 mg/kg, i.v.) could not be reversed by picrotoxin (1 mg/kg, i.v., twice with an interval of 10 min) or naloxone (1 mg/kg, i.v.). In parallel with convulsion, strychnine (1 mg/kg, i.v.) contracted the bladder which had been inhibited by baclofen and generated electrical activities consisting of efferent discharges and electromyograms. The dose of intracerebroventricularly or intrathecally injected baclofen which completely inhibited the bladder contraction was 0.1 or 10 micrograms, respectively. After the inhibition of bladder contraction by i.v. injection of baclofen, electrical stimulation of the sacral cord could contract the bladder and cause a fall in bladder pressure to around the level existing after micturition. From these results, the active site of baclofen which is related to the inhibition of bladder contraction is thought to be the micturition center in the brain stem.