1. The effects of prolonged exercise on energy metabolism in type I and type II muscle fibres in the vastus lateralis muscle were investigated in six male subjects (20.0 +/- 0.5 years, mean +/- S.E.M.) who performed one-legged cycling at 61% of maximum O2 consumption (VO2,max; determined with one leg) until fatigue or for a maximum of 2 h. 2. Analysis of pools of freeze-dried fibres obtained by needle biopsy and separated into specific types by the myofibrillar ATPase histochemical procedure indicated higher (P less than 0.05) lactate concentrations in type II fibres compared to type I fibres at 15 min (43.9 +/- 9.7 and 51.2 +/- 9.8 mmol (kg dry wt)-1) and at 60 min (18.2 +/- 4.7 and 25.9 +/- 6.5 mmol (kg dry wt)-1). No differences existed in lactate concentration between fibre types for pre-exercise (10.0 +/- 1.6 and 13.3 +/- 2.8 mmol (kg dry wt)-1) or post-exercise. 3. Glycogen degradation was most pronounced in type I fibres. By the end of exercise, glycogen concentration was 82.4 +/- 45 mmol glucosyl units (kg dry wt)-1 in type I fibres and 175 +/- 62 mmol glucosyl units (kg dry wt)-1 in type II fibres. 4. No significant changes in ATP and creatine phosphate (CrP) were found in either fibre type with exercise. 5. It is concluded that, at least for lactate and glycogen, fibre-specific differences are evident in prolonged submaximal exercise. The cause of the difference probably relates both to the unique energy metabolic characteristics of each fibre type and to the manner in which they are utilized during the exercise. 6. The failure to find a reduction in ATP concentration in either fibre type during prolonged exercise in the face of a progressive increase in the number of fibres showing little or no glycogen concentration suggests that protective mechanisms exist that prevent an energy crisis. The nature of these protective mechanisms remains to be elucidated.