A longitudinal in vivo and in vitro analysis of the genesis of insulin resistance has been carried out in mice made obese by chemical lesion (goldthioglucose, GTG) of the hypothalamus. Six weeks after GTG administration, glycemia and glucose disposal were normal but associated with increased insulin concentration, suggesting incipient insulin resistance. The in vitro counterpart of the latter in obese mice was observed in soleus muscle that was somewhat less responsive to insulin than controls, in liver that had increased basal lipogenesis but was uninfluenced by insulin, and in hepatic plasma membranes in which a slight decrease of insulin binding was measured. At this stage of obesity, basal adipose tissue lipogenesis was increased but the tissue responded in a normal fashion to insulin. These relatively discrete early metabolic changes were corroborated in vivo by a normal hypoglycemic effect of exogenous insulin. Sixteen weeks after GTG administration, hyperglycemia and gross hyperinsulinemia were recorded. This insulin resistance was evidenced in vivo by the lack of hypoglycemic effect of exogenous insulin unless considerable amounts of the hormone were administered. It coincided in vitro with a poor response of soleus muscle to insulin, an absence of a stimulatory effect of the hormone upon both adipose tissue and liver tissue, and a marked decrease in insulin binding to liver plasma membranes. It appears that insulin resistance is a multifactorial and progressive abnormality that might involve both insulin receptor and intracellular metabolic alterations. (Endocrinology102: 74, 1978)