In urethane-anesthetized rats the i.v. administration of the potent short-acting opioid, fentanyl, elicited inhibition of rhythmic spontaneous reflex increases in vesical pressure (VP) evoked by urinary bladder distension. The duration of fentanyl-induced inhibition was dose-related between 0.8 and 8.0 micrograms/kg i.v. Pretreatment with the opiate receptor antagonist, naloxone HCl (250 micrograms/kg i.v.), antagonized completely fentanyl's effect on VP rhythm. Results similar to those obtained with fentanyl were observed after the bilateral stimulation of opiate receptors in the dorsolateral tegmental-subceruleus region of the pons (DLT-SC region) with D-Ala2-Met5-enkephalinamide (DAMA) microinjections. These inhibitory effects were dose-related and significant between 1.0 and 100.0 ng/site. DAMA in the DLT-SC did not alter basal cardiorespiratory parameters. The systemic and local administration of naloxone reversed rapidly the VP effects elicited by DAMA in the DLT-SC region. Naloxone alone, administered either i.v. or locally in the DLT-SC region, increased the frequency of spontaneous reflex increases in VP. Moreover, pretreatment of the DLT-SC region with naloxone microinjections significantly (55% reduction) decreased the duration of fentanyl (8.0 micrograms/kg i.v.)-induced inhibition of VP rhythm. Neurons in DLT-SC region whose discharge increased just before and throughout spontaneous increases in VP (vesical contraction-related units), were inhibited rapidly and completely by the systemic administration of fentanyl. However, fentanyl elicited a prolonged decremental discharge in neurons of the DLT-SC region with discharge patterns related to vesical relaxation (vesical relaxation-related units).(ABSTRACT TRUNCATED AT 250 WORDS)