1. Peak isometric force of single fast (type 1) and slow (type 3) muscle fibres of Xenopus decreased when fibres were stimulated intermittently above their predicted sustainable duty cycle at 20 degrees C. Type 1 fibres could be fatigued to zero force. In most type 3 fibres force did not decrease below 50% of the original (P0) before activation failure, as indicated by irregular contractions. 2. Fibres were rapidly frozen at different force levels and analysed by high-performance liquid chromatography (HPLC) for ATP, IMP, phosphocreatine (PCr) and creatine (Cr). Lactate was determined enzymatically in type 1 fibres only. The relationships between force and PCr, and between force and ATP during fatigue were, apart from the range of values obtained, the same for both fibre types. When force had fallen to about 60-80% of original, PCr was fully reduced. At lower force levels, the ATP content-decreased, and a concomitant rise of IMP content was found. At zero force, ATP had fallen to about 25% of its value in rested type 1 fibres, and up to 200 mumol lactate (g dry weight)-1 had accumulated. 3. Recovery from fatigue was studied in fibres where force had fallen to 0.6 P0 (both fibre types) and 0.2 P0 (type 1 only). After 1 h of recovery ATP had in all cases returned to the level measured in rested fibres. In fibres fatigued to 0.6 P0, force almost returned to its original value. However, in type 1 fibres fatigued to 0.2 P0, it returned to only 0.3 P0. After 1 h of recovery the PCr/Cr ratio in type 1 fibres was lower (probability, P less than 0.05) than in control fibres, whereas in type 3 fibres it was not significantly different from controls. 4. The relationship between peak force and stimulus frequency, which had a sigmoid shape in fully rested fibres, was drastically changed by fatiguing stimulation. Immediately after fatiguing stimulation of type 1 fibres, force hardly increased with stimulus frequency, corresponding to the observation that calcium efflux from the sarcoplasmic reticulum was decreased at high stimulus frequencies. The force-frequency relationship of type 3 fibres was the same before and after intermittent stimulation.