The effect of diethylstilbestrol (DES) on bladder contractility was studied in vitro in 8 rats while 5 untreated animals were used as normal controls. Of the treated animals, 4 received 0.05 mg./day of DES during 8 weeks while another 4 were treated for 14 weeks with the same dose. We studied the animals by cutting consecutive rings from each bladder and testing them in a muscle chamber, under conditions of maximal electrical stimulation which had been previously determined in 3 animals. Parameters that produced maximal contractions were: 100 V, 40 Hz, and 2 msec. For each ring, we obtained full force-length relationships by stretching the rings in successive increments until length of maximal active force development was reached. For each bladder, alternate rings were tested in the muscle chambers while the remaining rings were used to determine collagen percentage with Woessner assay. In all animals, the ring with the largest diameter which was obtained from the widest portion of the bladder gave the highest contraction. The maximal active force of this ring was taken as a measure of contractility of the bladder. For controls, the maximal active force was 7.30 +/- 0.3 g. After 8 weeks of DES treatment, it decreased to 5.9 +/- 0.79 g and after 14 weeks, it further decreased to 2.15 +/- 0.99 g. The reduction in active force from controls after 8 and 14 weeks were statistically significant. Passive force and collagen content, however, did not change after either 8 or 14 weeks. In conclusion, these data show that DES decreases bladder contractility in male rats without affecting passive forces and muscle-collagen ratio.