Acetylcholine (ACh)-induced gallbladder smooth muscle contraction involves myosin light chain phosphorylation (MLCP). ACh-induced contraction is dose dependent. Whether MLCP by ACh is also dose dependent and how it correlates with contractile force have not been carefully evaluated. This study investigated the correlation between gallbladder muscle contraction and MLCP. Guinea pig gallbladder muscle strips were studied isometrically and frozen after different doses of ACh (0, 0.1, 5, 100 microM) for different periods of incubation (0.5, 1, 2, 3, 4 min). MLCP was determined using gel electrophoresis. Both contraction and MLCP in response to ACh were concentration dependent. Peak MLCP to ACh 100 microM occurred at 30 sec. There was a high correlation between active force and MLCP (r = 0.991; P = 0.009 at 30 sec stimulation). Nifedipine 1 microM reduced ACh-induced contraction and MLCP by a similar degree (31% and 33%, respectively). In conclusion, gallbladder contractile force significantly correlates with MLCP. This is consistent with the hypothesis that initiation of gallbladder cholinergic contraction is dependent on phosphorylation of myosin light chains.