The degradation of adenosine triphosphate, phosphocreatine and glycogen was determined in type I and type II fibres of the human quadriceps femoris muscle during intermittent electrical stimulation at 20 and 50 Hz, (1.6 seconds stimulation, 1.6 seconds rest). Seven healthy volunteers took part in the study. Muscle biopsy samples were obtained at rest and after 10 and 20 seconds of stimulation (six and 12 contractions, respectively). The resting contents of adenosine triphosphate, phosphocreatine and glycogen were all higher (P less than 0.05) in type II fibres compared to type I fibres. By the end of stimulation, whole muscle force production had declined to 84 and 77% of the initial force at 20 and 50 Hz, respectively. The phosphocreatine degradation rate for 10 and 20 seconds of stimulation was greater in type II fibres (P less than 0.05) compared to type I fibers at both 20 and 50 Hz. The rates of glycogenolysis after 20 seconds stimulation in type II fibres were 3.18 +/- 1.1 and 6.31 +/- 1.39 mmol glycosyl units kg-1 s-1. The corresponding rates in type I fibres were 0.46 +/- 0.73 and 0.60 +/- 0.39 mmol glycosyl units kg-1 s-1, which were not significantly different from zero. It is hypothesized that the decline in whole muscle force observed during electrical stimulation may be a consequence of the rapid loss of PCr stores in type II fibres.