The effects of administration of estradiol antiserum to pregnant rats on birth weight, subsequent growth velocity, carcass composition, and reproductive performance of their offspring were examined. Treated rats were injected i.p. with a potent antiestradiol serum on d 12 of gestation, whereas control rats received a similar injection of normal sheep serum. The pups from the treated rats were 12.2% heavier at birth than those from control rats (P less than .01) and had a greater postnatal growth rate (P less than .05). Chemical analysis of body composition at 56 d of age revealed that there was a tendency for female offspring from treated rats to exhibit carcass characteristics (such as increased percentage of body fat) that were more similar to those of normal male rats than to those of normal female rats. Neither onset of puberty nor reproductive performance was affected by the treatment. These results indicate that treatment of rats during pregnancy with antiestradiol may have potential as a technique for increasing postnatal growth rates.