We tested the in vitro effects of various glucose concentrations on the activity of hepatic pyruvate kinase, assayed at subsaturating, near physiological concentration (0.20 mmol/l) of the substrate phosphoenolpyruvate, to detect the "active" form of the enzyme. A 10-min incubation of mouse liver slices (n = 18) with increasing glucose concentrations (5, 10 and 20 mmol/l) resulted in a significant (p less than 0.01), progressive pyruvate kinase inhibition of 15, 28 and 41%, respectively. Similar data were obtained by incubating mouse liver homogenates (n = 7) with glucose, although with this material (which was supplemented with the pyruvate kinase activator fructose-1,6-diphosphate) the inhibition at the highest glucose concentration used was lower (24%, p less than 0.02). Addition of 10 nmol/l insulin during slice incubation (n = 8) prevented by 98% and 69% the inhibition exerted by 10 and 20 mmol/l glucose, respectively. Insulin alone was without effect on the enzyme activity. Glucose might inhibit pyruvate kinase by competing with the activator fructose-1,6-diphosphate. Insulin might overcome the glucose effect by activating pyruvate kinase through the known mechanism of enzyme dephosphorylation. Thus, in decompensated diabetes the high level of blood glucose may contribute, together with the counterregulatory hormones, to inhibit hepatic pyruvate kinase and therefore to stimulate gluconeogenesis.