The present investigation examines the association of angiotesin I converting enzyme (ACE) genotypes with blood pressure and heart weight in an F2 population of rats derived from a cross between spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. ACE genotype of rats in the F2 population was determined using a microsatellite polymorphism. Our investigation revealed that cardiac mass was not correlated with blood pressure at 12 weeks of age within the SHR, WKY, F1 or F2 groups of rats. In male rats, ACE genotype accounted for approximately 20% of the difference in mean blood pressure between SHR and WKY rats. There was no effect in females. It was also responsible for 21%-29% of the difference in heart weight both in female and male animals. The allele derived from the SHR parent appeared recessive to the allele from WKY parent for both heart weight and blood pressure. These results suggest that a gene in the region of the ACE locus is one of the genetic factors influencing blood pressure and heart weight in SHR.