Objective To observe the effect of selenium on the ACTH positive cells in the pars distalis of pituitary of rat with colon mucosa cancer induced by azoxymethane(AOM). Methods Twenty 3 weeks old Sprague Dawley male rats were randomly divided into 4 groups: the normal control group; the experimental control group; the selenium before AOM induced colon mucosa cancer group; the selenium after AOM induced colon mucosa cancer group. To induce rat colon mucosa cancer, 3 weeks old SD male rats received i.p. injections of AOM at a dose of 15mg/kg once a week for two weeks. Selenium (NaSUB>2/SUB>SeOSUB>3/SUB>, 4mg/L) began to be provided in the drinking water before and after AOM administration, and lasted for the whole experiment period. The rat pituitary tissues were excised from the 34 weeks old rats. The morphological changes of the ACTH positive cells in the pars distalis of pituitary were observed using immunohistochemical methods， and then the results were analyzed by image analysis system. Results Aberrant crypt foci (ACF) and aberrant crypt (AC) were showed in the colon mucosa of rats that received two weekly i.p. injections of AOM by light microscopy with methylene blue staining. The ACTH positive cells in the experimental group rat displayed a significant increase compared with those in the normal control group rat (P<0.01) with immunohistochemical methods. There were more positive intensity in the cells with ACTH positive cells in the supplement selenium group than that in the experimental group (P<0.01). Conclusion Selenium can promote the function of the ACTH positive cells in the pars distalis of rat pituitary with colon mucosa cancer ind