Tonic smooth muscle exhibit the latch phenomenon: high force at low myosin regulatory light chains (MRLC) phosphorylation, shortening velocity (Vo), and energy consumption. However, the kinetics of MRLC phosphorylation and cellular activation in phasic smooth muscle are unknown. The present study was to determine whether Ca2＋-stimulated MRLC phosphorylation could suffice to explain the agonist- or high K＋-induced contraction in a fast, phasic smooth muscle. We measured myoplasmic [Ca2＋], MRLC phosphorylation, half-time after step-shortening (a measure of Vo) and contractile stress in rabbit urinary bladder strips. High K＋-induced contractions were phasic at both 22oC and 37oC: myoplasmic [Ca2＋], MRLC phosphorylation, 1/half-time, and contractile stress increased transiently and then all decreased to intermediate values. Carbachol (CCh)-induced contractions exhibited latch at 37oC: stress was maintained at high levels despite decreasing myoplasmic [Ca2＋], MRLC phosphorylation, and 1/half-time. At 22oC CCh induced sustained elevations in all parameters. 1/half-time depended on both myoplasmic [Ca2＋] and MRLC phosphorylation. The steady-state dependence of stress on MRLC phosphorylation was very steep at 37oC in the CCh- or K＋-depolarized tissue and reduced temperature flattend the dependence of stress on MRLC phosphorylation compared to 37oC. These data suggest that phasic smooth muscle also exhibits latch behavior and latch is less prominent at lower temperature.