Pregnant rats became significantly heavier than non-pregnant controls 2 days after conception. Just prior to term, they exhibited a gross 52% increase over their non-pregnant body weight. Dry kidney weight had increased by 13% in 7-day-pregnant animals, which also showed a 20% increase in proximal renal tubule lengths. At term, the former was not maintained as compared with controls; whereas the latter is maintained throughout pregnancy, and may, at least partially, account for the enhanced renal reabsorption of salt and water found in these animals.