In precollicular decerebrate cats and dogs the intravenous administration of naloxone reduced urinary bladder capacity. Successive cystometrograms revealed that naloxone in doses of 10-100 micrograms/kg i.v. reduced the volume necessary to evoke micturition by 21-67% (mean 48%) in cats and 15-81% (mean 43%) in dogs, respectively. Microinjection of fentanyl (0.4-10 nM) into the pontine micturition center (PMC) increased the bladder capacity by 4-46% (mean 18%) in cats. Naloxone injected into the same site reversed the effect of fentanyl. Microinjection of naloxone (40-120 nM) into the PMC reduced the bladder capacity by 17-57% (mean 34%) in cats. These data indicate that endogenous opioid peptides may have a role in controlling micturition in both decerebrate cat and dog, and that the enkephalinergic inhibitory mechanisms are important in modulating the micturition reflex at the level of the pontine micturition center.