In order to study the potential influence of rectal enemas on epithelial cell proliferation, two groups of 13 patients without any disease of the digestive tract were compared, one group having received two warm enemas (Normacol) prior to proctoscopic examination. Cell proliferation was studied by in vitro incorporation of tritiated thymidine in mucosal biopsies and radioautographic analysis of the number and position of labeled nuclei in the glands. The mean number of labeled cells per gland and the mean labeling index were significantly higher in group i (with enemas) than in group II (without enema). In group I there was an extension of the proliferative compartment towards the surface with less than 50 p. 100 of labeled cells located in the lower third of glands. Intraindividual variations of labeling index from gland to gland were more important in group I than in group II. In group I, eight out of 13 patients had rectal glands with elevated labeling indices (15 p. 100) in contrast to one patient in group II. Our results suggest that proliferation abnormalities observed in group I are related to enema administration and indicate that this type of rectal preparation should preferably be excluded whenever cell kinetic parameters are studied in the rectal mucosa.