Leptin levels are significantly elevated in pregnant mice, rats and humans suggesting a critical role for leptin during gestation. To address whether leptin plays a putative role in the physiology of pregnancy, we asked whether a mouse pregnancy would be affected by the complete absence of leptin from both the mother and fetuses. Thus, leptin-deficient ob/ob females were first treated with exogenous leptin and then mated to similarly treated ob/ob males. All resulting fetuses have an ob/ob genotype and lack like their mothers any endogenous leptin production. Withdrawal of leptin treatment at 0.5, 6.5, 10.5 and 19.5 days p.c. did not affect any stage of the pregnancy despite a gradual return of the mothers to an obese state. However, some mice had delayed gestation periods of 21–23 days which were associated with prolonged parturition. The pups were normally delivered with no obvious signs of deformities although none survived beyond a day after delivery due to failure of lactation. Monitoring daily food intake of pregnant ob/ob females treated throughout gestation with leptin revealed significantly elevated levels of food intake from day 10 p.c. and onward demonstrating an attenuation of a leptin response during pregnancy and a leptin resistance effect. These studies demonstrate that in the mouse, leptin is not a critical molecule for implantation, gestation, fetal growth and parturition but that the leptin resistance effect at mid-gestation aims to stimulate food intake thus providing sustained energy resources for pregnancy.