A histological assessment has been made of both time- and dose-related changes in the choroid plexus after the local irradiation of the rat brain with single doses of 17.5-25 Gy of X rays. These investigations involved the serial killing of animals 1-52 weeks after irradiation and the quantitative and semiquantitative evaluation of histological sections. Counts of the relative number of cells in the choroid plexus of the lateral ventricles showed an atrophy of the epithelial layer after 13 weeks. However, this was not as marked as the reduction in the number of endothelial cells in the wall of blood vessels. Moreover, the epithelium had recovered by 39 weeks after irradiation, while the dose-related depletion in endothelial cells tended to be progressive. A highly correlated group of changes in the vascular-connective tissue was used to produce a numerical "factor". This represented a combined score of radiation damage which was both time- and dose-related. These data suggest that, as an expression of late radiation damage to the choroid plexus, the effect on the endothelium was more important than that to the epithelial cells.