The influence of 400 R X-irradiation on the localization and the number of mature and immature goblet cells and Paneth cells in rat duodenal epithelium has been studied. At short times after irradiation, when the total proliferative activity in the crypts of Lieberkuhn is reduced, the proportion of mature and immature goblet cells of the total number of crypt cells was increased; also an absolute increase in the number of goblet cells in the crypts was found. The immature goblet cells were localized in the lower half of the crypt as in control animals, whereas the number of the mature cells increased over the whole crypt length. When the proliferative activity of the crypt cells increases again from 12 to 48 hr after irradiation the number of both types of goblet cells decreases. Between 48 and 72 hr, when the whole crypt is involved in proliferation, a second increase of both types of goblet cells was found. However, the localization of the immature goblet cells is no longer restricted to the lower half of the crypt but they also appear at the higher cell positions. On the villus no immature goblet cells were found and the changes in the numbers of mature goblet cells do reflect the changes induced by irradiation in the goblet cell population in the crypt. The absolute number and localization of Paneth cells did not change under the experimental conditions. The findings are discussed in relation to cell proliferation and differentiation processes in intestinal crypts.