The uptake of [3H] uridine into the nucleic acids of rat uterus during early pregnancy.

1. Changes in content and uptake of [(3)H]uridine into the nucleic acids of rat uterus during the first 9 days of pregnancy were studied. 2. From day 6 implantation sites were separated from the rest of the uterine tissue for independent analysis. 3. Up to day 5 of pregnancy no changes were found in the total dry matter or in RNA and DNA content/unit dry matter nor in the RNA/DNA ratios. 4. From day 6, when implantation sites are visible, the water content of the implantation sites increased by 2-3%, and the RNA content/unit dry wt. and the RNA/DNA ratios increased. The DNA content/unit dry wt. did not increase in the implantation sites until day 8. 5. Uptake of [(3)H]uridine into the acid-soluble fraction of the tissues was markedly higher in implantation sites than in non-implantation sites. 6. Uptake of [(3)H]uridine into RNA was significantly increased on day 3 of pregnancy and again on day 5. 7. On days 6 and 7, the incorporation into RNA of implantation sites was significantly higher than in the remainder of the uterine tissue but decreased on days 8 and 9 to the same value as that of the normal tissue. 8. No change occurred in uptake into DNA until day 6, when there was an increase in uptake by the implantation sites. 9. It is suggested that the increase in RNA synthesis on day 3 is a preparation of the uterus for the onset of implantation on day 5, and that increased synthesis in implantation sites on days 6 and 7 is the elaboration of new RNA necessary for this early stage of pregnancy to commence.