To search for the genes that determine the level of blood pressure, we performed linkage analyses using rat microsatellite markers in two sets of F2 rats derived from two hypertensive rat strains and one normotensive rat strain. In the first F2 progeny obtained from spontaneously hypertensive rats and Wistar-Kyoto rats, it was found that two different loci cosegregated with blood pressure. One was gamma-crystallin locus on rat chromosome 9, which has a hypertensive effect, and the other was peroxisomal 3-keto acyl-CoA thiolase locus on rat chromosome 8, which has a hypotensive effect. Then, we studied the same loci in second F2 progeny obtained from Dahl/Iwai salt-sensitive rats and Wistar-Kyoto rats. It was found that the locus located on rat chromosome 8 also had a hypotensive effect, although the others were not cosegregated with blood pressure in this cross. We suggest that a new locus for hypertension is located on rat chromosome 9 and that a locus on rat chromosome 8 has a common effect that makes blood pressure decrease in different hypertensive strains.