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Department of Physiology, Medical College of Oita, Japan.

1. Maintenance heat produced in tetani of frogs' sartorius muscles (Rana japonica) was measured under various values of intracellular pH (pHi) brought about by increasing the CO2 concentration in Ringer solution. The pHi values were measured using 31P nuclear magnetic resonance from the chemical shifts of the inorganic phosphate resonance. The pHi was 7.10 +/- 0.009 (mean +/- S.E. of the mean, n = 10) in the gas mixture of 5% CO2/95% O2 at 4 degrees C and it was reduced to 6.44 +/- 0.001 (n = 23) in 45% CO2. 2. As CO2 was increased, the maximum force was decreased and relaxation was prolonged. This is in accordance with the results of Edman & Mattiazzi (1981) and Curtin (1986). 3. An increase in CO2 induced a reduction of the maintenance heat production, which can be divided into stable and labile heats (Aubert, 1956). The stable heat, which is produced with a steady rate during contraction, was decreased as CO2 was increased. The labile heat, which is produced with an exponentially declining rate, was not significantly altered by increasing CO2 within the range studied. 4. The effect of previous contractile activity on the labile heat production, i.e. the time course of repriming of the labile heat, could be described by an equation with two exponential terms in 5% CO2 in accordance with the result of Peckham & Woledge (1986). The time course of repriming of the labile heat was not affected by increasing CO2 to 20%.