Energy balance of pregnant and non-pregnant female rats was determined after 20 days of gestation. Energy contents of male and female pups and placentae were also determined. The metabolizable energy intake of pregnant rats was 33% higher than that of non-pregnant controls. Body weight gain was 9.4-fold that of non-pregnant rats. Gains in energy and body weight in pregnant rats exceeded those of pups and placentae. This implies that the excess energy was stored, probably for later use in an energy-requiring process such as lactation. Food was processed as expected in pregnant rats, since although gross food efficiency (gain in body energy divided by metabolizable energy intake) was greater in pregnant than in non-pregnant rats (5.4% vs 17.5%), only 53.7% of the extra energy intake was deposited in the dams.