It is well known that the VO2 readjustment rate of the whole body is faster when carrying out a given constant work load starting from a baseline of moderate exercise than from rest. However, it has not been established whether the above change is the result of faster kinetics of the oxidative machinery or, alternatively, the consequence of a reduced involvement of confounding factors such as anaerobic glycolysis or tissue O2 stores. The problem, earlier approached by chemical methods, was studied in man by 31P-NMRS assessment of the kinetics of phosphocreatine (PC) hydrolysis at the muscle level which is known to reflect the readjustment rate of the oxidative reactions. Twelve normal subjects carried out in a 90 cm bore modified Picker (1.5 T) magnet, a series of contractions by the plantar flexors reaching pre-set submaximal loads either in single steps (constant load, CL) or progressively (incremental exercise. I). If preceding exercise (I), compared to rest, influenced the rate of oxidations, the PC concentration at the target loads would be different for the two exercise modes, reflecting different energy deficits. This was not the case. Thus the present results show that the rate of readjustment of oxidations at the muscle level is not affected by priming exercise confirming previous findings and showing that theoretical models of VO2 control are experimentally applicable to man.