1. Glycogenolysis in type I and II muscle fibres was investigated in five healthy volunteers during electrical stimulation of the quadriceps muscle group with blood flow occluded. 2. The quadriceps femoris muscles were stimulated intermittently (1.6 s stimulation, 1.6 s rest) at a frequency of 50 Hz for 64 s and isometric contraction force was recorded. Muscle biopsies were obtained at rest prior to and immediately after stimulation. Single muscle fibres were dissected free and were identified as type I and II fibres. ATP, phosphocreatine (PCr) and glycogen contents were measured luminometrically and enzymatically in single fibres and mixed fibre muscle. 3. Electrical stimulation resulted in a marked decline in contraction force and near total depletion of PCr in both fibre types. The ATP turnover rate (P < 0.05) and the magnitude of the decline in ATP (P < 0.05) were greater in type II fibres. Prior to stimulation the muscle glycogen content was 32% higher in type II fibres compared with type I fibres (P < 0.01). During stimulation the rate of glycogenolysis in type II fibres (4.32 +/- 0.54 mmol (kg dry matter (DM)-1 s-1 was twofold greater than the rate in type I fibres (2.05 +/- 0.70 mmol (kg DM)-1 s-1, P < 0.05). 4. The data suggest that the relatively higher rate of glycogenolysis observed in type I fibres during intermittent electrical stimulation with occluded circulation (2.05 +/- 0.70 mmol (kg DM)-1 s-1), when compared with the corresponding rate recorded during intense contraction with circulation intact (0.18 +/- 0.14 mmol (kg DM)-1 s-1, P < 0.05), may result from an accelerated ATP turnover rate in this fibre type increasing the cellular concentrations of free AMP and inosine 5'-monophosphate (IMP), which are known activators of glycogen phosphorylase. 5. The similarity in the rate of type II fibre glycogenolysis during contraction with circulatory occlusion (4.32 +/- 0.54 mmol (kg DM)-1 s-1), when compared with the corresponding rate recorded during non-occluded circulation (3.54 +/- 0.53 mmol (kg DM)-1 s-1, P > 0.05), is in agreement with the suggestion that glycogenolysis in this fibre type is already occurring at a near-maximal rate with circulation intact.