1. S A is a recently identified gene implicated in blood pressure regulation in rodent models of genetic hypertension. In this study we have examined, by Northern blotting, its expression in tissues of the spontaneously hypertensive rat, the Wistar-Kyoto rat and F 2 rats, derived from a cross of the spontaneously hypertensive rat with the Wistar-Kyoto rat. 2. We demonstrate that the gene is expressed in a tissue-specific manner. Expression was detected in four sites: kidney, liver, brain and testes. 3. In the kidney and liver expression was higher in the spontaneously hypertensive rat than in the Wistar-Kyoto rat, whereas in the brain and testes the pattern was reversed. 4. In the F 2 rats, the levels of S A mRNA in the liver, brain and testes were found to be primarily determined by the genotype at the S A gene locus. 5. The findings suggest the presence of strain-specific cis - and more than one tissue-specific trans -acting factors regulating the expression of the rat S A gene.