The present study was undertaken to elucidate aspects of the regulatory mechanisms leading to enhanced glucose metabolism and insulin sensitivity of muscle after physical exertion. Biopsies were obtained from the vastus lateralis muscle of healthy volunteers before and after 60 min of bicycle exercise at 60% of their maximal aerobic capacity. Insulin binding to wheat germ agglutinin-purified muscle insulin receptors as well as basal and insulin-stimulated receptor kinase activity toward an exogenous substrate were unaltered by exercise. Muscle glycogen levels diminished from 3.35 +/- 0.26 to 1.85 +/- 0.13 mg/100 mg muscle (P less than 0.01) and the half-maximal activation constant of glycogen synthase for glucose 6-phosphate decreased from 0.62 +/- 0.05 to 0.25 +/- 0.02 mM (P less than 0.001). Total glycogen synthase activity was unchanged. In the absence of phosphatase inhibitors, glucose 6-phosphate-independent glycogen synthase activity of the crude enzyme extract increased during in vitro incubation. The initial rate of activation (through dephosphorylations) of glycogen synthase was 0.18 +/- 0.06 vs. 0.37 +/- 0.03 U.min-1.mg-1 protein before and after exercise, respectively (P less than 0.02). The total as well as the glycogen-associated phosphoprotein phosphatase activity was, however, unaffected by exercise.