Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto ("WKY") rats are frequently employed in experimental studies of hypertension. Although both SHR and WKY rat strains have been presumed to be fully inbred, recent studies have revealed important biologic variability in WKY rats from different commercial sources. Genealogic evidence suggests that, in the United States, breeding stocks of WKY rats may have been distributed to major commercial suppliers as early as the F10 generation. To test the hypothesis that commercially available WKY rats are genetically heterogeneous, we performed deoxyribonucleic acid (DNA) "fingerprint" analysis on genomic DNA of WKY rats from two of the largest vendors in the United States, Taconic Farms and Charles River Laboratories. We found molecular evidence of genetic variability not only among WKY rats from two different breeding facilities, but also among WKY rats within a single breeding facility (Taconic Farms). Although some studies have suggested the possibility of biologic variability in SHR from different sources, preliminary studies have not revealed molecular evidence of genetic heterogeneity in SHR from these vendors. In demonstrating genetic variability in WKY rats from different sources, the current study provides compelling evidence that rats designated WKY do not constitute an inbred strain. Accordingly, the results of studies in which SHR and WKY rats are compared might vary because of genetic heterogeneity in "the WKY rat control strain."