The purpose of this study was to examine the effect of ramp slope during progressive exercise test on the intracellular pH threshold (pHT) of working muscle. To attain the purpose, we measured intracellular pH of wrist flexor by 31P-MRS during four kinds of ramp wrist flexion. Five Japanese males participated as subjects. They performed wrist flexion in a bore of MRS system, with ramp increments of 0.14, 0.20, 0.26 and 0.31 W/min till exhaustion. Intracellular pH of wrist flexor muscle was estimated from a chemical shift between inorganic phosphate (Pi) and phosphocreatine (PCr) on a 31P-MR spectra. Pi-to-PCr ratio (Pi/PCr) as an index of oxidative phosphorylation potential was also calculated. pH and Pi/PCr were obtained as minute-by-minute data throughout the exercise. pHT was determined as a work rate (W) just before that at which the intracellular acidosis (acidemia) occurred. Although work endurance was shortened significantly, achieved maximal work rate (WRmax) was increased with an increase in ramp slope. pHT both in the unit of W and %WRmax was not affected significantly by the ramp slope. On the other hand, Pi/PCr at pHT decreased with an increase in ramp slope, significantly. These results suggested that intracellular homeostasis with reference to acid-base equilibrium was not dependent on oxidative phosphorylation status, but a work output of the muscle.