The genetically obese-hyperglycemic mouse, C57 BL/6J-ob, exhibits hyperglycemia and hyperinsulinemia. We have investigated the in vivo hepatic response to a glucose load in female obese mice and their lean littermates. Within 15 min after the administration of glucose (1.5 g/kg) to fasted lean mice, gluconeogenesis from [14C]alanine markedly decreased, endogenous hepatic levels of alanine and other gluconeogenic precursors increased, and glycogen synthetase was activated by virtue of an increase in the precent of synthetase I. These changes persisted up to 60 min and then returned to fasting values. In contrast, obese mice did not produce any of the above changes when given a similar glucose load. Failure to activate glycogen synthetase occurred despite the presence of synthetase D phosphatase activity. In lean mice [14C]glucose synthesis from [14C]glycerol exceeded that from [14C]alanine and was not suppressed by glucose administration, indicating the site of control of gluconeogenesis to be below the triose phosphate step. Insulin resistance in obese mice may involve the liver, as well as peripheral tissues studied by others.