The parameters P(isv) (active isovolumetric detrusor pressure) and Vmax (maximum shortening velocity), which characterize the contractility of the detrusor muscle, were determined in guinea pigs. To this end it was necessary to develop a method of measuring flow rates in these small animals. The values found were used to calculate the contractility parameter Wmax. Thirteen animals were used. The results found for P(isv) and Vmax were 43.0 +/- 3.7 cm. H2O and 20.2 +/- 3.7 mm. per second, respectively. The latter corresponded to about 0.38 muscle lengths per second, which is similar to values reported for bladder strips from other species. Previous work showed that in vitro P(isv) decreased with increasing bladder volume over a wide range of volumes. In vivo P(isv) seemed to be independent of bladder volume. This suggests that neurogenic stimulation intensifies as volume increases. Vmax also was independent of volume. Wmax appeared to be suitable for detecting differences in the contractility of the bladders of different animals. Values were not significantly different in isovolumetric and nonisovolumetric contractions. Normalized to the size of the bladder, the Wmax values indicated that the power generated by the guinea pig bladder is similar to the power generated by the human bladder.