Heat production during and after contraction of isolated rabbit papillary muscles was measured at 20°C with metal-fllm thermopiles. Under control conditions (0.2 Hz, pH 7.4) total heat and steady-state force production due to 120 twitches were 1.0 ± 0.4 J/g and 29.5 ± 5.1 mN/mm2(mean ± SD;n=5), respectively. Increasing the CO2of the bicarbonate-buffered superfusate from 5% (pH 7.4) to 24% (pH 6.6) led to a decrease of force and heat production, 54% and 72%, respectively, of the control value. The recovery heat-time constant, reflecting the time course of oxidative phosphoryiation, increased from 23.0 ± 5.1 seconds at pH 7.4 to 69.5 ± 34.7 seconds at pH 6.6. The ratio of recovery and initial heat equaled 1.06 under both conditions. Enhancing the metabolic rate by increasing the stimulation frequency to 1.0 Hz led, after an initial maximum, to a decline of force and heat presumably as the consequence of shortage of oxygen in the muscle core. The recovery phase in this case was characterized by a double exponential function having time constants of 7.6 and 64.4 seconds. When pH was lowered to 6.6 together with the enhancement of the stimulation frequency to 1 Hz, an additional exothermal process, unrelated to force production, was observed during contraction and for some time thereafter. It was concluded that severe acidosis slows down the rate of oxidative phosphoryiation and may reduce the economy of contraction. However, it does not change the nature of recovery and initial heat processes. These data suggest the possible mechanism of an additional exothermal process occurring at a high stimulus frequency and low pH.