The present study was designed to evaluate the effects of changes in glycogen concentrations on lactate accumulation in human skeletal muscle during a 1 min maximal muscle fatigue test (MFT). All subjects first performed the MFT during a control experiment. Four subjects then performed the MFT again 1 h after prolonged exercise. Seven other subjects performed the MFT again after a 3 day carbohydrate (CHO) poor diet and again after 4 additional days of a CHO rich diet. The m. vastus lateralis was biopsied prior to (for glycogen determinations) and immediately after (for lactate determinations) each performance of the MFT. High but similar lactate concentrations were observed (22.3 mmol x kg-1 w.w.) with normal and supernormal glycogen levels. Lactate was significantly reduced following both the prolonged exercise (to 7.0 mmol x kg-1 w.w.) and the CHO poor diet (to 16.8 mmol x kg-1 w.w.). Alterations in muscle strength and fatigue patterns were also observed from the dietary manipulations but they were neither commensurate with the changes in metabolite concentrations nor statistically significant.