Basal and postprandial metabolic rates were determined by indirect calorimetry in lactating, nonlactating postpartum, and nulliparous women who received a controlled diet of protein at 1.0 g.kg-1.d-1 and energy that approximated usual intake levels on the basis of diet records. Milk production was measured by the test-weighing procedure and by milk expression. Basal metabolic rates (BMRs) were similar among all groups of women. No relationships were detected between BMRs and postpartum time in lactating women. In contrast, postprandial metabolic rates (PMRs) increased (P less than 0.01) in lactating compared with nonlactating women. Milk nitrogen concentrations but not energy showed a linear relationship with PMR (r = 0.86, P less than 0.001). The energy cost of milk production was estimated to be 125% of the energy output in milk. Thus, increased energy needs during lactation are associated not only with milk production but also with elevated PMRs. This increase was met by an increase in dietary energy intakes of 50% above the intakes of nonlactating counterparts.