The incorporation of tritiated thymidine, uridine, and leucine into normal colonic cells, cells of excrescences, and mucosa adjacent to excrescences was studied after incubation in vitro and microautoradiography. In normal mucosa, epithelial cells synthesizing DNA and proliferating in the lower two-thirds of colonic crypts incorporate more uridine and leucine than mature surface cells. In hyperplastic mucosa in excrescences, in some areas of mucosa adjacent to excrescences and occasionally in histologically normal mucosa in patients without excrescences, greater incorporation of uridine and leucine was observed in cells lining the luminal surfaces of the colonic crypts. Chase experiments, designed to follow newly formed and labeled RNA, demonstrated a shift of nuclear RNA to cytoplasm within 1–3 hours in normal cells. A more rapid nuclear-to-cytoplasmic RNA shift in hyperplastic cells and a slower shift in villous papilloma cells compared to normal cells suggested differences in the kinetics of rapidly labeled RNA in these lesions. These changes were accompanied by decreases in acid and alkaline phosphatases.