Aspects of the control of glycolysis during contractile activity were investigated in the phasic adductor muscle of the bay scallop, Argopecten irradians concentricus. Glycogen phosphorylase and phosphofructokinase displayed relatively low maximum catalytic activities in this tissue. Comparison of mass action ratios to apparent equilibrium constants showed that both enzymes were displaced from equilibrium under resting conditions. During contractile activity, both enzymes moved toward equilibrium, indicating a large increase in catalytic activity. However, this increase in activity was accompanied by only a slight increase in substrate levels where there was a substantial increase in product levels. Both of the activities of phosphorylase and phosphofructokinase appeared to be enhanced by factors other than substrates/products and therefore can be viewed as being important control sites. The elevated glycolytic rate was manifested by a substantial accumulation of octopine during contractile activity. Kine...