Force-velocity relations obtained by the quick release technique were studied at different times during the twitch at 25 degrees C in electrically stimulated longitudinal smooth muscle from rabbit urinary bladder. The results show that maximal shortening velocity (Vmax) and maximal tension (Po) reach their highest values during the rising phase of the twitch. Maximum Vmax was calculated to be 0.30 +/- 0.04 1/s (S.E., n -= 9). Vmax declines rapidly with time and at the peak of the twitch it is only about 70% of the maximum value. On the other hand Po at the peak does not differ significantly from its maximum value during the rising phase. In the relaxation phase of the twitch, both Vmax and Po decrease progressively. Photographic studies of preparations marked with charcoal grains did not show any considerable inhomogeneity of contraction during isometric twitches, maximum movement of grains never exceeding 4% of total muscle length. No yield at the ends of the preparations were seen. The different time course of Vmax and Po can therefore not be explained by longitudinal inhomogeneity of activation or deactivation of the preparations.