This investigation is designed to critically examine the possibility that conformational changes experienced by myosin play a relevant role in the contraction mechanisms. To accomplish this goal, the mechanism of skeletal and cardiac myosin ATPase will be studied by fluorescence stopped-flow techniques. Fluorescence probes and fluorescent substrate will be used to elucidate the kinetic steps involved in the ATP hydrolysis. Energy transfer within the enzyme-substrate complex and between myosin and actin during the course of hydrolysis will be measured by both stopped-flow and nanosecond fluorescence methods. It is our hope that the kinetic data when considered along with new structural information from nanosecond studies, will provide a more realistic scheme for the elucidation of the contractile mechanism.