In the present study, the effects of age and diet on glucose disappearance and tissue-specific glucose uptake (R'g) were examined under basal or hyperinsulinemic, euglycemic conditions in male Sprague-Dawley rats. Rats were equicalorically fed either a high-starch diet (68% of kcal), high-fat diet (HFD; 45% of kcal), or high-sucrose diet (68% of kcal), beginning at either 5 (W; weanling), 10 (Y; young), 18 (M; mature), or 58 wk (O; older) of age for 5 wks (n = 6-9 · group¹ · diet¹). Body weight gain was not significantly different among dietary groups within a given age. Significant (P < 0.05) age effects were observed on basal and clamp free fatty acid concentrations. Significant diet effects were observed on basal and clamp triglyceride concentrations. There were significant diet and age effects on basal skeletal muscle R'g. This interaction was primarily due to an age-associated increase in basal R'g (µg · g¹ · min¹) in HFD (gastrocnemius R'g: 0.9 ± 0.2 in W, 1.1 ± 0.2 in Y, 1.8 ± 0.2 in M, 2.5 ± 0.2 in O). Both age and diet significantly decreased insulin-stimulated muscle R'g. However, whereas age-associated reductions in both glucose-6-phosphate concentration and glycogen synthase activity were observed, significant diet effects were observed on glucose-6-phosphate concentrations only. Age significantly reduced basal and clamp adipose tissue R'g when expressed per gram of tissue but significantly increased R'g when expressed per total fat pad mass. These data suggest that diet-induced changes in peripheral glucose metabolism are modulated by age.