In urethane-anesthetized rats with an intact spinal cord, application of capsaicin on the outer surface of the urinary bladder produced a transient bradycardia, hypotension and negative cardiac inotropism which were neither prevented by i.v. atropine (0.5 mg/kg) nor by cervical vagotomy. In acute spinal rats (C2-C3) application of capsaicin (0.2 and 2 micrograms in 25 microliters) on the urinary bladder induced a transient hypertension, tachycardia and positive cardiac inotropism. A second application (30 min later) induced minor cardiovascular effects, expecially with the higher dose, indicating desensitization. All cardiovascular responses to topical capsaicin were abolished by systemic capsaicin desensitization (50 mg/kg s.c., 4 days before). The excitatory cardiovascular response to capsaicin in acute spinal rats was markedly reduced by bilateral section of pelvic but not hypogastric nerves. Further, it was abolished by pretreatment with hexamethonium (20 mg/kg i.v.) or reserpine (5 mg/kg i.p., 2 days before) and reduced, at various extent for the different components, by phentolamine (0.5 mg/kg i.v.) or propranolol (1 mg/kg). In rats with pelvic and hypogastric nerves intact, section of the cord at a level (T12-L1), just above the medullary segments which receive primary afferent input from the bladder (L6-S1), abolished the excitatory cardiovascular response to application of capsaicin on the bladder. In spinal rats (C2-C3) rapid distension of the urinary bladder with saline produced transient tachycardia, hypertension and positive cardiac inotropism similar to that evoked by capsaicin.(ABSTRACT TRUNCATED AT 250 WORDS)