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A B S T R A C TIn a gcncration time of 8 hr in Paramecium caudatum, the bulk of D N A synthesis detected by thymidine-3H incorporation takes place in the latter part of the cell cycle. The micronuclear cycle includes a G1 of 3 hr followed by an S period of 3 -3~ hr. G2 and division occupies the remaining period of the cycle. Macronuclear R N A synthesis detected by 5'-uridine-3H incorporation is continuous throughout the cell cycle. Micronuclear R N A synthesis is restricted to the S period. Ribonuclcase removes 80-90 % of the incorporated label. Pulsc-chasc experiments showed that part of the R N A is conserved and releascd to the cytoplasm during the succecding G1 period.