The anterior latissimus dorsi (ALD) of chicks (2 wk old) was examined for rate of heat liberation when the muscle was reversibly stretched from L0 to about 1.6 L0. The "plateau" forces of isometric tetani were related to the corresponding rates of heat liberation. Presumably, in this region of length change the degree of overlap between the thick and thin filaments is changing. If the amount of overlap is known, the number of heavy meromyosin cross bridges in activity can be estimated. From this information, the amount of liberated energy can be calculated for a single cross-bridge cycling. In all of our measurements, there was heat production even at muscle lengths so great that no filament overlap would have to be assumed. This finding was incorported into the estimate for energy release per cross-bridge cycling.