The dynamics of lactate accumulation in working muscle was studied in three subjects performing maximal bicycle exercise of 2, 6, and 16 min duration. In separate experiments, the two longer maximal work periods were interrupted after 2 min and after 2 and 6 min, respectively. Biopsy specimens from the quadriceps femoris were obtained immediately after the work was terminated for determination of ATP, CP, glycogen, G-6-P and lactate. Blood lactate was also determined. The breakdown of the phosphagens (ATP and CP) was already maximal after 2 min of work in all experiments and averaged 2.7 and 3.6 mmole kg–1 wet muscle, respectively. The accumulation of lactate in the muscle and in the blood increased continuously until exhaustion and averaged in the muscle tissue both at the highest and medium loads 16.1 but was only 12.0 mmole kg–1 wet muscle at the lowest load. It is concluded that low ATP and CP stores in these experiments was not the reason for muscular fatigue. Further, if the muscle tissue lactate concentration was the reason for exhaustion on the two heaviest work loads another factor must be present to explain the exhaustion in the 16-min experiment. exhaustion; muscle metabolites; muscle lactate; blood lactate; anaerobic metabolism; oxygen deficit