The in vivo response to radiotherapy is not well understood but appears to involve the p53 tumor suppressor protein. We investigated the expression of apoptosis-inducing p53 target genes during gamma-irradiation-induced cell death in p53(+/+) or p53(-/-) mouse tissues using in situ hybridization. Our results reveal striking tissue specificity with distinct regulation of target p53-induced genes in different cells and tissue compartments, as well as variations in dependence on p53 for basal expression. p53-dependent induction of Puma occurred in the splenic white pulp, whereas Noxa and Bid were induced in the red pulp. These patterns correlated with activation of caspase-3 in both compartments. All apoptotic targets of p53 studied here (DR5, Bid, Puma, Noxa) were induced in the jejunum and ileum, which appeared to be the tissues most sensitive to irradiation. We also observed unexpected differences in p53 target gene activation between the transverse and descending colon. Finally, in the liver where irradiation did not lead to caspase-3 activation, we primarily observed p21(WAF1) induction as the major p53-dependent target gene response. Our findings indicate that the selectivity of p53 in transactivation following DNA damage in vivo results in unique tissue and cell type specificity, which may correlate with growth arrest or variable sensitivity to gamma-irradiation.