In obese hyperglycemic (ob/ob) mice, as compared to controls, hepatic pyruvate kinase (PK) activity was enhanced by 35.63% (214.75 +/- 13.60 nmol/min/mg protein v 158.33 +/- 10.47, P less than 0.01) when measured at saturating (6.6 mmol/L) concentration of the substrate phosphoenolpyruvate (total activity), but the activity recorded at subsaturating (1.3 mmol/L) substrate concentration (active fraction) was unchanged (86.37 +/- 6.42 v 85.66 +/- 13.59) or even decreased if expressed as percent of the total activity (40.21 +/- 2.56% v 54.10 +/- 5.07, P less than 0.05). Since insulin induces the synthesis of hepatic PK and favors the conversion of the inactive (phosphorylated) to the active (dephosphorylated) form, these findings suggest that in ob/ob mice the striking hyperinsulinemia, although it is able to increase the hepatic content of PK, fails to activate this enzyme. This may favor gluconeogenesis in these animals. The hepatic concentration of PK effectors (fructose-1,6-P2 and phosphoenolpyruvate) was unchanged in ob/ob mice, and the in vitro effect of the activator fructose-1,6-P2 (15 mumol/L), which would favor the activation (dephosphorylation) of PK, was preserved. It is suggested that hepatic PK in ob/ob mice is resistant to activation by insulin.