The renin gene is known to be overexpressed in the spontaneously hypertensive rat (SHR) of the Okamoto strain. As the first intron of many genes controls transcription rate, we examined whether the first 1,100 base pairs of the SHR first intron possessed mutations in putative transcriptional factor binding sites. Such mutations might then form the basis for overexpression of the renin gene in the SHR. A BglII restriction fragment length polymorphism (RFLP) was identified in the first 1,100 base pairs of the first intron of the renin gene of the SHR when compared to Wistar Kyoto (WKY) and Sprague Dawley (SD) normotensive rats. Sequence analysis of this region located the BglII RFLP between positions 501-505 of the rat renin gene. The new BglII cut site was produced by a single base mutation from G to A at position 502. While a number of other insertional and deletional events were found in the SHR, WKY and SD sequences over this region, only two were unique to the SHR. These mutations occurred at positions 191, 502, 934 and 1070. The latter three fell within sequence motifs known to bind the transcriptional factors PPAR, E2A and AP2 respectively. Thus we propose that these mutations alter the DNA binding characteristics of one or more transcriptional factors to the SHR renin gene first intron resulting in its overexpression which, in turn, might form the basis for a tissue renin-angiotensin dependent hypertension in this strain.