To define the role of the liver in whole body glucose homeostasis, we generated mice with a liver-specific knockout of the insulin receptor (IR) gene using the Cre-loxP system. Mice carrying an altered IR gene with exon 4 flanked by loxP sites were bred with mice that express Crc recombinase from the albumin promoter/enhancer. The resultant liver-specific IR knockout (LIRKO) mice were born with the expected frequency and had the same body mass at birth as control littermates. Interestingly, for 3-5 days pest-weaning, LIRKO mice gained less weight than controls, but this was corrected by 6 weeks of age due to a transient increase in the growth rate. On gross examination, the livers of LIRKO mice were about 50% of normal size, and liver IR content, as measured by western blotting, was reduced by 85%, similar to the percentage of liver represented by hepatocytes. At 2 months of age, male LIRKO mice were hyperglycemic in the fed state as compared to IR (lox/lox) controls (363 [+ or-] 52 vs. 132 [+ or-] 2.8 mg/dl), yet after a 15 hr fast, the LIRKO mice were only mildly hyperglycemic as compared to IR (lox/lox) controls (132 [+ or-] 10.1 vs. 95 [+ or-] 3.3 mg/dl). Serum insulin levels in LIRKO males were elevated 20-fold in the fed state (78.5 [+ or-] 6.9 vs. 3.9 [+ or-] 0.53 ng/ml). 53 ng/ml) and 14.3-fold after a 15 hr fast (4.6 [+ or-] 0.17 vs. 0.32 [+ or-] 0.2 ng/ml) as compared to IR (lox/lox) controls. Preliminary histological analysis of the pancreas indicated that LIRKO mice have a significant increase in islet size. Intra-peritoneal (IP) insulin tolerance tests (0.75 U Humulin R/kg body weight) performed at 2 months of age demonstrated that LIRKO mice were completely resistant to the blood glucose lowering effects of exogenously-administered insulin. Similarly, IP glucose tolerance tests (2 g glucose/kg body weight) performed at 2 months of age demonstrated severe glucose intolerance in LIRKO mice. Somewhat surprisingly, by 4 months of age the fasting hyperglycemia had normalized, although fasting and fed hyperinsulinemia, fed hyperglycemia, insulin resistance and glucose intolerance remained. In addition, serum triglycerides and tinge fatty acids were suppressed by 30-50% in 4 month old male LIRKO mice as compared to control groups. These studies indicate that isolated liver insulin resistance is sufficient to cause severe defects in glucose and lipid homeostasis, but not uncontrolled fasting hyperglycemia or diabetes. This defect also leads to hyperinsulinemia, likely due to changes in both insulin secretion and insulin clearance.