Qualitative and quantitative morphological changes in rat jejunum were studied after a whole-body exposure to 3 Gy of gamma rays. Four groups of animals were irradiated at different times of the day, namely midnight, 06.00, 12.00 and 18.00 hours. The number of epithelial cells, labelling and mitotic indices were evaluated in crypt sections and the spatial distribution of S-phase cells was determined. At 12 h after irradiation a marked reduction was observed in all parameters, but the proliferative activity was restored quickly and at 36 h after irradiation the values were significantly higher than the controls. The frequency distribution of labelled cells at different positions in the crypt was reduced at 12 h but a clear expansion of S phase cells to positions near to the crypt villus junction was observed during the recovery phase. The animals irradiated at different times of the day showed a similar general post-irradiation response in the number of cells along the side of the crypt, labelling and mitotic indices and in the distribution of S phase cells along the crypts. It is worth noting that the animals exposed at midnight had a distribution of S phase cells similar to controls at 72 h post-irradiation, i.e. earlier than the other groups.