Isolated rat hepatocytes were incubated in the absence or presence of glucagon and the activity of fructose-1,6_bisphosphatase was measured in cell extracts. After glucagon treatment the Vmax was increased (20-50s) whereas the K,,, remained unchanged. The stimulation was complete at 5 min after addition of glucagon. The glucagon concentration needed for maximal stimulation was 10m9M. After gel filtration the fructose-1,6-bisphosphatase activity in extracts of glucagon-treated cells was lowered to the control level. The effect of glucagon could not be completely mimicked by dibutyryl CAMP. The data indicate that in addition to the possible regulatory role of enzyme phosphorylation, a positive effector is involved in the stimulation of fructose-1,6_bisphosphatase activity by glucagon.Fructose-l,&bisphosphatase Glucagon Low-M, activator (Rat hepatocyte)