It was found that increase in foetal weight in albino rats in the 3rd week of pregnancy was accompanied by an increase in the dry weight of placenta and uterine wall and a decrease in the water levels of foetus and placenta. It was not correlated with the increased water levels of the uterine wall. Of the increase in maternal body weight not due to weight of uterus, some 7% was due to increased weight of gastro-intestinal contents and the remainder to increase in dry weight or water levels, or both, of a series of body organs such as skeletal muscle, skin, liver, spleen, ovaries and adrenal glands. Increase in weight of body organs was not positively correlated with the very considerable increase in weight of concepta. Hypertrophy and hydration of maternal body organs did not appear to be due, therefore, to metabolic demands associated with the marked increase in conceptal weight during the third week. Increase in weight of the foetus was associated with a decline in the weight of certain organs such as pyloric stomach. The weight of the ovaries declined as placental weight increased.