Pregnant rats underfed in the first 2 weeks of gestation had offspring with normal body weight at birth and weaning. However, starting at about 5 weeks of age the male offspring became hyperphagic and gained more weight than the controls. The female offspring did not overeat and did not become obese. Both male and female progeny showed increased fat cell size as adults. In males the epididymal and retroperitoneal fat pads were significantly enlarged nd adipocytes in these pads were hypertrophied. In females and fat pads were not enlarged, but adipocytes in the parametrial pads were hypertrophied.