One of the most prevalent lesions in DNA is the apurinic/apyrimidinic (AP) site, which is derived from the cleavage of the N-glycosyl bond by DNA glycosylase or by spontaneous depurination. AP sites are repaired by AP endonucleases during the process of base excision repair; however, an imbalance in this DNA repair system may cause mutations as well as cell death. We have established a sensitive and convenient slot-blot method to detect AP sites in genomic DNA using a novel aldehyde reactive probe (ARP), which reacts with the aldehydic group of ring-opened AP sites. The reaction of 1 mm of ARP with 15 µg of genomic DNA containing AP sites at 37°C was completed within 1 min. The AP site-ARP complex was remarkably stable during incubation in TE buffer, even at 100°C for 60 min. The sensitivity of this assay enables detection of 2.4 AP sites per 10⁷ bases. By using this ARP-slot-blot assay, the rate of spontaneous depurination of calf thymus DNA was determined. Under physiological conditions, AP sites were increased at 1.54 AP sites/10⁶ nucleotides/day (9000 AP sites/cell/day). This highly sensitive assay allows us to determine the endogenous level of AP sites in genomic DNA, as well as to investigate whether DNA-damaging agents cause imbalances of base excision/AP endonuclease repair in vivo and in vitro.