The behaviour of intracellular pH (pHi) was studied with micro-electrodes in frog semitendinosus muscle which was superfused with Ringer solution and with depolarizing solutions. The electrodes were introduced into the depolarized muscle about 40 min after contracture had subsided. All studies were done at external pH (pHo) of 7.35 and at 22 degrees C. The pHi in normal Ringer solution buffered with HEPES was 7.18 +/- 0.03 (S.E. of mean) (n = 10); the membrane potential, Vm, was -88 +/- 1.8 mV. When pHi was lowered to about 6.8 by replacing the HEPES by 5% CO2, 24 mM-HCO3 (constant pHo), it recovered at a very slow rate of 0.025 +/- 0.011 delta pHi h-1 (n = 6). When all the Na was replaced by N-methyl-D-glucamine (initial pHi 7.20 +/- 0.04, initial Vm -89 +/- 1.5 mV, n = 8), this slow alkalinization was converted into a slow acidification at a rate of 0.069 +/- 0.024 delta pHi h-1. In muscle depolarized in 15 mM-K (Vm approximately -50 mV), the rate of recovery from CO2 acidification was not increased above that in normal Ringer solution (2.5 mM-K). When, however, the muscle was depolarized in 50 mM-K to about -20 mV, the rate of recovery increased to 0.33 +/- 0.07 delta pHi h-1 (n = 6) when external Cl was kept constant, or to 0.21 +/- 0.03 (n = 9) when [K]. [Cl] product was kept constant. In the absence of Na, pHi recovery rate in 50 mM-K was reduced by at least 90%. Enhanced recovery from CO2-induced acidification was also observed in 2.5 mM-K when the fibres were depolarized to about -20 mV in one of two ways: (a) by previous exposure for 60 min to 50 mM-K at constant Cl, or (b) by reduction of external Cl to 5.9 mM in the presence of 0.5 mM-Ba. When pHi of depolarized fibres (50 mM-K) was lowered to about 6.8 by the weak acid dimethyl-2,4-oxazolidinedione (DMO), it recovered at a rate of 0.12 delta pHi h-1 in two experiments. In fibres depolarized in 50 mM-K and constant Cl, either 0.1 mM-SITS or 0.5 mM-amiloride slowed pHi recovery from CO2 exposure by about 50%. When the depolarization was achieved at constant [K]. [Cl] product, amiloride slowed pHi recovery by about 50%, while SITS had, at most, only a slight effect.(ABSTRACT TRUNCATED AT 400 WORDS)