The present investigation was performed to evaluate the possible role of arginine vasopressin (AVP) in detrusor instability in the obstructed rat urinary bladder. The effect of AVP on normal and obstructed rat detrusor smooth muscle was tested in vivo and in vitro. Arginine vasopressin given as a closed intraarterial injection to the bladder transiently decreased micturition volume and increased micturition frequency during cystometry in control rats. In rats with infravesical outlet obstruction the effect of AVP on cystometrical parameters was negligible. In accordance with this finding, the efficacy of AVP in contracting detrusor muscle in vitro was much lower for obstructed bladders than for controls. The EC50 values were, however, similar. Arginine vasopressin added to the bath had no effect on nerve-mediated contractile responses. Obstruction led to a transient decrease in immunoreactive AVP concentration, but the total amount of AVP per bladder increased significantly after 6 weeks of obstruction as a consequence of the 14-fold increase in bladder weight. The decreased excitatory effect of AVP in obstructed bladder makes a role for this peptide in the development of detrusor instability less likely.