We have assessed the ability of compound 48/80, a mast cell degranulating agent, to activate the sensory and efferent function of capsaicin-sensitive primary afferents in the rat urinary bladder. Compound 48/80 produced a calcium-dependent release of calcitonin gene-related peptide-like immunoreactivity from the superfused rat urinary bladder. This effect was prevented by in vitro capsaicin desensitization, but was not affected by indomethacin, methysergide, ondansetron, chlorpheniramine or cimetidine, nor by systemic pretreatment with compound 48/80 at a dose regimen which prevented lethality produced by intravenous administration of it in anesthetized rats. Compound 48/80 also produced a contraction of the rat isolated bladder which was not reduced by methysergide, indomethacin or in vitro capsaicin desensitization. In vivo, topical application of compound 48/80 on the serosal surface of the rat urinary bladder activated a series of high amplitude rhythmic bladder contractions which were hexamethonium-sensitive (micturition reflex). This effect was prevented by systemic capsaicin desensitization while it was unchanged by chlorpheniramine, methysergide, indomethacin or ondansetron. Administered intravenously, compound 48/80 produced a plasma protein extravasation (Evans blue leakage technique) in the rat urinary bladder which was abolished by systemic capsaicin pretreatment or chlorpheniramine while it was unaffected by methysergide or indomethacin. The present findings provide direct neurochemical evidence that compound 48/80 activates the peripheral endings of capsaicin-sensitive primary afferent neurons, leading to a stimulation of their sensory and efferent functions in the rat urinary bladder. The possibility of a direct action of compound 48/80 in producing excitation of capsaicin-sensitive sensory nerves should be considered.