Abstract
1. The increase in rate of heat production caused by stretch in the unstimulated frog's sartorius (stretch response) has been measured using a conventional thermopile technique.2. The rate of heat production was found constant between l(0) (the distance in vivo between the tendons when the legs were in a straight line) and 1.2 l(0), and rose rapidly above this length to reach 3-5 times the basal rate at 1.3 l(0). Stretching to greater lengths appeared to damage the muscles.3. The stretch response is increased by several substances which increase the duration of the active state.4. Unlike the rate of heat production at l(0), the stretch response is increased by procaine; while the presence of CO(2) greatly reduces it.5. Evidence is presented supporting the hypothesis that the stretch response is associated with the appearance of tension in the sarcolemma.