In single twitches of toad sartorii at 0 degrees C, released at constant speed by an ergometer, the relation has been studied experimentally between total heat production and velocity of shortening. The curve is complex, first falling and then rising with increasing speed. The reasons for the form of the curve are discussed. Carlson, Hardy & Wilkie (1963 a, b) reported recently that in after-loaded isotonic twitches the heat production remained about constant when the load was varied. They concluded that in the complete cycle of a twitch there is no evidence for heat of shortening. In the present experiments, in which the speed of shortening was varied instead of the load, the results were quite different. The heat altered considerably and the heat of shortening was obvious. An explanation of the contradiction is offered. Mean curves are given for the heat H, the total energy (H + W) and the efficiency W/(H + W) as functions of the velocity of shortening in a twitch. An Appendix refers to Fenns original experiments (1923).