The metabolic intermediate concentrations were determined and the responses of phosphofructokinase (PFK) activity to physiological changes of the intermediate concentrations were investigated in the dorsal muscle of carp Cyprinus carpio. Each concentration of ATP, ADP, AMP, G6P, F6P, FDP, and citrate in the muscle ranged from 10−3 M to 10−4 M. PFK showed allosteric characteristics under the physiological pH (6.8) and F6P concentration(0.2×10−3 M) and its activity was inhibited by high ATP concentrations such as the physiological range. However, its inhibition was reduced by the presense of ADP or AMP, while it was reinforced by that of citrate at physiological concentration ranges. The results suggest that in vivo glycolytic activity can be regulated not only by enzyme induction but also by PFK activity without enzyme induction through changes in the physiological concentrations of metabolic intermediates as substrate and effector.