Glucose metabolism was studied in anesthetized lactating rats in the postabsorptive state. Basal levels of blood glucose and plasma insulin were lower in 12-day-lactating rats than in age-matched nonlactating rats. When the pups were removed for 24 h, the maternal blood glucose level reached a value intermediate between lactating and nonlactating values, and the plasma insulin level was the same as in nonlactating rats. Glucose turnover was increased from 3 days postpartum on in lactating rats compared with nonlactating rats. At peak lactation (12-19 days) glucose turnover was 80% higher in lactating than in nonlactating rats. In the lactating rats weaned for 24 h, glucose turnover returned to the value of the nonlactating rats. Insulin secretion in response to an intravenous glucose load (IVGTT) was not modified in lactating rats compared with nonlactating rats but was increased threefold in weaned rats. This suggests that nonlactating tissues are insulin resistant during lactation. During euglycemic hyperinsulinemic clamp, glucose clearance was increased threefold in lactating and in nonlactating rats and twofold in weaned rats, suggesting that glucose metabolism in the mammary gland is affected by insulin. Measurement of lipogenesis gave direct evidence for the insulin responsiveness of the mammary gland and for the insulin resistance of adipose tissue during lactation.