Rat fetuses were subjected to selective hypophysectomy in utero on day 16, 17 or 18 of gestation and inspected on day 22 (the day before birth). The survival rate of fetuses operated on day 17 and 18 was about 20 and 50%, respectively. Losses were mostly caused by the surgery preceding the removal of the pituitary, and no significant survival-promoting activity of the gland could be demonstrated. The surgical injuries, as such, also reduced the weight increase substantially. Still, the loss of the pituitary was followed by a further reduction in weight gain, which on day 22 was about 10% lower than that of the sham-operated fetuses.