The purpose of this study was to compare the intramuscular and the intravascular events in relation to energy metabolism during progressive arm exercise. Twelve healthy untrained Japanese males participated in this study as subjects. They performed wrist flexion in a ramp incremental load of 0.14 W/min until exhaustion. 31P-MR spectra were obtained from wrist flexor muscle before and throughout the exercise. Venous blood was also sampled from antecubital vein with one minute interval during the exercise, and a change in plasma lactate concentration (La) was observed. Intracellular pH (pH) was calculated from a chemical shift between phosphocreatine (PCr) and inorganic phosphate (Pi) of the 31P-MR spectra. Change in pH showed a threshold behavior during exercise. Threshold points at decline in pH (pHT), increase in Pi/PCr (PT), and increase in La (LT) were determined by piecewise linear regression analysis of minutes-by-minutes data. Mean values of pHT, PT and LT were 43.0, 42.5, and 24.8% of maximal work rate, respectively. LT was significantly smaller than pHT and PT. This result suggests that lactic acidosis has already existed when pH is kept at resting level, and pHT reflects the capacity of remaining intracellular biochemical homeostasis, which might be one of the physiological indices of muscle fatigue.