Vacuoles develop after fatiguing stimulation in frog skeletal muscle fibres. Experiments on isolatedXenopus muscle fibres show that this vacuolation is a dynamic process that reaches its maximum about 20min after the end of fatiguing stimulation and then recedes. Fatigue-induced vacuoles originate from the t-tubular system. Recent data indicate that vacuoles are formed because of lactate accumulation in the t-tubules resulting in increased osmotic pressure and subsequent water influx. There is no obligatory connection between the presence of vacuoles and force depression, which is another common feature during the recovery from fatigue. Nevertheless, extensive vacuolation may exaggerate this force depression.