We evaluated insulin sensitivity in epididymal adipocytes from two mouse strains shown to be either sensitive (AKR/J, n = 14) or resistant (SWR/J, n = 12) to the development of obesity when fed a high-fat diet. Half of each strain was fed a chow (CH) diet (12% fat), and half received a sweetened condensed milk (CM) diet (33% fat). After 1 wk, epididymal adipose depots were removed and digested with collagenase, and glucose transport was measured with labeled 2-deoxyglucose. Plasma glucose and insulin were slightly higher in AKR/J than SWR/J mice (glucose: 139.7 vs. 118.8 mg/dl, P < 0.06; insulin: 3.45 vs. 2.99 ng/ml, P < 0.04). One week of high-fat feeding increased adipose depot mass and carcass lipid in both strains to approximately the same extent. Adipocytes from AKR/J mice had greater insulin-stimulated glucose transport compared with SWR/J mice at both submaximal and maximal insulin levels (P < 0.0001). Short-term feeding of the high-fat diet increased AKR/J adipocyte insulin sensitivity but decreased the sensitivity of SWR/J adipocytes to insulin. The differences in adipocyte insulin sensitivity between strains were not explained by differences in adipocyte cell size. Access to the high-fat CM diet for 12 wk increased total dissected adipose depot size by 209% in the AKR/J mice and 82% in the SWR/J mice. These data clearly demonstrate that the two strains differ in adipocyte insulin sensitivity as well as sensitivity to dietary obesity. Increased adipocyte insulin sensitivity could contribute to a predisposition to increase adipose tissue lipid stores with diets high in fat content.