In a generation time of 8 hr in Paramecium caudatum , the bulk of DNA synthesis detected by thymidine- 3 H incorporation takes place in the latter part of the cell cycle. The micronuclear cycle includes a G 1 of 3 hr followed by an S period of 3–3½ hr. G 2 and division occupies the remaining period of the cycle. Macronuclear RNA synthesis detected by 5'-uridine- 3 H incorporation is continuous throughout the cell cycle. Micronuclear RNA synthesis is restricted to the S period. Ribonuclease removes 80–90% of the incorporated label. Pulse-chase experiments showed that part of the RNA is conserved and released to the cytoplasm during the succeeding G 1 period.