The renin genes of the spontaneously hypertensive rat (SHR) and Wistar-Kyoto (WKY) rat were compared by Southern blotting using cDNA and oligonucleotide probes. A deletion of 650 base pairs was found in the first intron (intron A) of the SHR gene compared with the WKY gene. Our studies strongly suggest that this is due to a decrease in the number of copies of the tandemly repeated sequence present within intron A of the rat renin gene. In both SHR and WKY, this region of the gene was found to be different from that of the parent Wistar rat and those of other Wistar-based inbred strains. The functional significance of the abnormality and any role it may have in hypertension in the SHR remain to be determined.