Total protein, RNA and DNA content and the activity of acid and alkaline phosphatases, 5´-nucleotidase and isocitrate dehydrogenase were studied in rat uterus during the first 8 days of pregnancy. Isocitrate dehydrogenase activity showed marked fluctuations from day to day. Nucleotidase and acid phosphatase activities showed a significant increase on day 8. The most marked change in activity was that of alkaline phosphatase which showed a 10-fold increase between days 6 and 8, due largely to an increase in the activity of this enzyme in the decidual nodule. The rise in alkaline phosphatase activity did not occur in rats ovariectomized on days 1, 2 or 4 of pregnancy and was markedly decreased in those ovariectomized on day 6. [<sup>3</sup>H]-uridine incorporation into RNA showed a significant increase between days 2 and 6 whereas [<sup>3</sup>H]-thymidine incorporation into DNA showed a significant increase on day 6.