The time course of phosphocreatine (PC) hydrolysis in humans was measured by 31P-NMR spectroscopy (31P-NMRS) with a time resolution of 10.8 s in the gastrocnemius muscle and a relationship between muscle O2 consumption (VO2) and [PC] was derived from a bioenergetic model. This allowed a direct estimate of the half-time of the intracellular VO2 kinetics (t1/2 VO2) of the contracting human gastrocnemius in aerobic conditions. t1/2 VO2 was found to be approximately 16 s and independent of the work load. This value corresponds to the shortest t1/2 VO2 determined at the mouth of the subject in the absence of lactate accumulation in the rest to work transient. t1/2 VO2 may now be assessed in man at low muscle temperatures. To this aim a procedure was developed allowing corrections of the 31P-NMR spectra based on the muscle temperature profiles obtained by a simultaneously acquired proton image.