We have implemented two computation versions of the solution actin-myosin ATPase cycle. A simple numerical model that simulates an eight step ATPase cycle and allows actin, myosin, substrate ATP and product ADP & Pi concentrations to be fixed or varied. This model allows us to rapidly assign realistic rate constants to each step of the cycle for any well-defined myosin isoform. With this set of rate constants in place we can then implement a stochastic version of the eight step ATPase cycle that includes calcium regulation of the actin binding steps according to the McKillop & Geeves Blocked/Closed/Open model of the regulated thin filament. Using this model it is now feasible to model the dependence of the ATPase rate on calcium concentration and predict the occupancy of each state in the cycle for any defined set of protein and substrate concentrations. This allows us, for example, to explore the interplay of calcium and ATP concentrations in the cooperative activation of the ATPase cycle and make predictions for how the system will respond to sudden changes in calcium or ATP concentration. In a further variant of the model the program allows us to predict how the system will respond if variable ratios of fast and slow isoforms of myosin, or variable ratios of proteins carrying cardiomyopathy mutations are present in the system. This will be a useful tool to explore the behaviour of mixed myosin & troponin populations and correlate experimental data with model predictions.Supported by: R01 AR048776, R01 DC 011528 and W.Trust 085309