Muscle metabolites and blood lactate concentration were studied in five male subjects during five constant-load cycling exercises. The power outputs were below, equal to and above aerobic (AerT) and anaerobic (AnT) threshold as determined during an incremental leg cycling test. At AerT, muscle lactate had increased significantly (p less than 0.05) from the rest value of 2.31 to 5.56 mmol X kg-1 wet wt. This was accompanied by a significant reduction in CP by 28% (p less than 0.05), whereas only a minor change (9%) was observed for ATP. At AnT muscle lactate had further increased and CP decreased although not significantly as compared with values at AerT. At the highest power outputs (greater than AnT) muscle lactate had increased (p less than 0.01) and CP decreased (p less than 0.01) significantly from the values observed at AnT. Furthermore, a significant reduction (p less than 0.05) in ATP over resting values was recorded. Blood lactate decreased significantly (p less than 0.01) during the last half of the lowest 5 min exercise, remained unchanged at AerT and increased significantly (p less than 0.05-0.01) at power outputs greater than or equal to AnT. It is concluded that anaerobic muscle metabolism is increased above resting values at AerT: at low power outputs (less than or equal to AerT) this could be related to the transient oxygen deficit during the onset of exercise or the increase in power output. At high power outputs (greater than AnT) anaerobic energy production is accelerated and it is suggested that AnT represents the upper limit of power output where lactate production and removal may attain equilibrium during constant load exercise.