We created gradual partial urethral obstruction in 20 guinea pigs using silver jeweler's jump rings. After 4 or 8 weeks obstruction all animals underwent cystometry and were assigned to one of five urodynamic categories: normal, high pressure voiding, unstable, low compliance, or decompensated. After sacrifice, the contractile responses of bladder strips to electrical field stimulation of intramural nerves, direct electrical muscle stimulation, 0.1 mM carbachol, and high K+ solution were sampled by computer for phase plot analysis. Following 8 weeks obstruction, the value of the phase plot parameter Fiso, indicative of the number of contractile muscle units, was reduced to 60% of the control response to nerve stimulation (P < 0.05) and to 77% of the control response to carbachol stimulation (P < 0.05). Parameter C, the slope of the phase plot (indicative of unit recruitment during force development), was unchanged for all forms of stimulation. Although in the latter case not statistically significant, obstruction affected responses to nerve and muscle stimulation similarly suggesting that muscle change may possibly be a common denominator of dysfunction. In view of the reduction in Fiso and the increase in bladder weight, instability may represent a more advanced form of dysfunction due to obstruction than high pressure voiding.