Intestinal epithelial cell proliferation in young male F344 rats was measured in response to dietary protein content (5%, 10%, and 20% casein diets), energy restriction (energy intake was 60% of ad libitum energy intakes of animals consuming the 20% casein diet), total diet restriction (dietary intake was 60% of the ad libitum intake of 20% casein diet group), aflatoxin administration, and whole body irradiation. Cellular proliferation was measured in sections of jejunum, ileum, proximal colon, and distal colon with the [3H]thymidine technique. Restricting energy or total diet intakes by 40% from ad libitum levels reduced proliferation in epithelial cells throughout the intestine. In comparison to the 5% casein diet, the 20% casein diet resulted in modestly lower cellular proliferation in all intestinal segments. Radiation induced a decrease in cellular proliferation in the jejunum and ileum; this decrease was prevented by a 20% casein diet. Pretreatment with aflatoxin B1 decreased intestinal cell proliferation throughout the intestine, and this decrease was not influenced by the protein content of the diet.