1. The performance of skeletal muscle during repetitive stimulation may be limited by the development of an intracellular acidosis due to lactic acid accumulation. To study this, we have measured the intracellular pH (pHi) with the fluorescent indicator BCECF (2',7'-bis(carboxyethyl)-5(6)- carboxyfluorescein) during fatigue produced by repeated, short tetani in intact, single fibres isolated from the mouse flexor brevis muscle. 2. The pHi at rest was 7.33 +/- 0.02 (mean +/- S.E.M., n = 29, 22 degrees C). During fatiguing stimulation pHi initially went alkaline by about 0.03 units (maximum alkalinization after about ten tetani). Thereafter pHi declined slowly and at the end of fatiguing stimulation (tetanic tension reduced to 30% of the original; 0.3Po), pHi was only 0.063 +/- 0.011 units (n = 14) more acid than in control. 3. We considered three possible causes of acidosis being so small in fatigue: (i) a high oxidative capacity so that fatigue occurs without marked production of lactic acid; (ii) an effective transport of H+ or H+ equivalents out of the fibres; a high intracellular buffer power. 4. The oxidative metabolism was inhibited by 2 mM-cyanide in three fibres. After being exposed to cyanide for 5 min without stimulation, the tetanic tension was reduced to about 0.9 Po and pHi was alkaline by about 0.1 units. The fibres fatigued faster in cyanide and the pHi decline in fatigue was more than twice as large as that under control conditions. 5. Inhibition of Na(+)-H+ exchange with amiloride resulted in a slow acidification of rested fibres; resting pHi was not affected by either inhibition of HCO3(-)-Cl- exchange with DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) or inhibition of the lactate transporter with cinnamate. 6. Fibres fatigued in cinnamate displayed a markedly larger acidification (approximately 0.4 pH units) and tension fell more rapidly than under control conditions; inhibition of Na(+)-H+ and HCO3(-)-Cl- exchange did not have any significant effect on fatigue. 7. The intracellular buffer power, assessed by exposing fibres to the weak base trimethylamine, was about 15 mM (pH unit)-1 in a HEPES-buffered solution (non-CO2 or intrinsic buffer power) and about 33 mM (pH unit)-1 in a bicarbonate-buffered solution. Somewhat higher values of the intrinsic buffer power was obtained from changes of the partial pressure of CO2 (PCO2) of the bath solution. Application of lactate or butyrate frequently gave an infinite buffer power, which indicates that powerful pH-regulating mechanisms operate in these cases.(ABSTRACT TRUNCATED AT 400 WORDS)