Intravesical pressure recordings of the urinary bladder in anesthetized rats were performed and the role of substance P (SP) in the motor control of this organ was evaluated. Regional injection of SP (0.4 nmoles i.a.) into the superior vesical artery elicited a prompt bladder contraction; this motor response was dosedependent. The detrusor contraction could be completely inhibited by a SP-analogue, (D-Pro2, D-Trp7,9)-SP (45-90 nmoles i.a.). Furthermore, the detrusor contraction evoked by preganglionic stimulation of the pelvic nerves was partially inhibited by the same antagonist in a higher dose (65% reduction at a total dose of 150-300 nmoles). The contractile response to SP (0.5 nmoles i.a.) was also significantly reduced after blockade of muscarinic receptors with atropine (50% reduction at 1 mg/kg i.a.) or after ganglionic blockade with hexamethonium (75% reduction at 25 mg/kg i.v. + 50 mg/kg hr i.a.). Immunocytochemical studies demonstrated the occurrence of SP-immunopositive nerve terminals in the detrusor part of the rat urinary bladder. Based on these findings it is suggested that SP may act as a neurotransmitter/modulator in this organ. The mechanism of action for SP on the detrusor seems to be complex and may involve ganglionic transmission via both types of cholinoceptors as well as direct activation of smooth muscle.