Feeding a high fat diet during lactation should reduce the competition for glucose utilization between the mammary gland and the other maternal tissues, because dietary fat is directly utilized for milk lipid synthesis. Glucose homeostasis was studied in nonlactating and lactating rats fed a high fat diet and compared with that in rats fed a high carbohydrate diet. In nonlactating rats fed a high fat diet, blood glucose concentration was slightly higher, whereas plasma insulin concentration was lower than in nonlactating rats fed a high carbohydrate diet. In the postabsorptive state, plasma free fatty acids and blood ketone bodies were not modified by the nature of the diet consumed. Glucose turnover rate and glucose metabolic clearance rate in the postabsorptive state in nonlactating rats fed a high carbohydrate diet were not different from those in rats fed a high fat diet [9.5 +/- 1.4 vs. 8.8 +/- 0.8 mg/(min X kg) and 8.9 +/- 1.2 vs. 7.9 +/- 0.7 mL/(min X kg)]. In lactating rats, blood glucose and ketone bodies, plasma insulin and free fatty acid concentrations were not affected in the postabsorptive state by the composition of the diet consumed. The glucose metabolic clearance rate in lactating rats fed the high carbohydrate diet was higher than that in nonlactating rats fed the same diet. However, the glucose metabolic clearance rate in lactating rats fed the high fat diet was not different from that in nonlactating rats fed the same diet.