Intracellular pH (pHi) has been measured before fatiguing stimulation and during recovery in single toe-muscle fibres of Xenopus at room temperature. Liquid ion-sensor microelectrodes were used for pHi measurements. The pHi measured before fatiguing stimulation was 6.93 +/- 0.11 (mean +/- SD, n = 9) in type 1 fibres and 6.99 +/- 0.10 (n = 4) in type 2 fibres. About 1 min after tension had been suppressed to approximately 40% of the original by repeated tetanic contractions, pHi was measured again; it was then reduced to 6.34 +/- 0.13 (range 6.15-6.50) and 6.71 +/- 0.17 (range 6.50-6.85) in type 1 and type 2 fibres, respectively. The pHi recovered at a rate of about 0.05 pH units min-1 and was always normalized well before tension. Fibres which exhibited post-contractile depression (PCD), a delayed force suppression during the recovery period, had similar pHi normalization rates to those of other fibres. The large variation in pHi values obtained in fibres fatigued to a standard tension level leads us to conclude that an intracellular acidification is not likely to be the major cause of fatigue produced by intermittent tetanic stimulation. However, an important inhibitory effect in the most acidified fibres, cannot be excluded. Furthermore, we conclude that force recovery in our experiments is controlled by factors other than pHi.