The p53 tumor suppressor protein mediates cell cycle arrest and apoptosis through transactivation of downstream target genes. While many target genes have been identified to date, the mechanisms and time course of their induction are still unclear. We investigated the kinetics of p53 binding to the p21CIP1, MDM2, BAX and PIG3 promoters in vivo using a novel quantitative real-time chromatin immunoprecipitation-PCR assay. Our results demonstrate distinct kinetics of p53 promoter binding dependent on the target gene promoters. The timed induction of target genes due to genotoxic stress is likely to play a pivotal role for the divergent functions of p53.