Very low incorporation of 3H-thymidine (TdR) into neurons and non-proliferating glial and endothelial cells in various brain areas of the adult mouse after 3H-TdR injection and subsequent X-irradiation of the head with 45 Gy has been demonstrated autoradiographically after exposure times of 250 days. In accordance with biochemical studies this incorporation of 3H-TdR represents DNA repair synthesis or UDS (unscheduled DNA synthesis). However, 3H-TdR incorporation into nuclear DNA of non-proliferating cells in the brain was not only found in X-irradiated but also in sham-irradiated mice. This suggests that spontaneous UDS also occurs. Up to now spontaneous UDS has been shown only in HeLa cells in vitro. Nearly all the various types of brain cells tested exhibited UDS after X-irradiation as well as spontaneous UDS. After correcting the mean grain numbers per nucleus not only for background but also for beta-self-absorption, substantial differences became apparent in the extent of UDS between the individual types of cells. After X-irradiation, UDS was highest in Purkinje cells and hippocampal granular cells but comparable UDS was also found in endothelial cells, regardless of the different brain areas studied. The extent of spontaneous UDS is also quite different in the various cell types, being highest in neurons of different sites and considerably lower in endothelial and glial cells.