The spontaneously hypertensive rat (SHR) has a Y chromosome locus that increases blood pressure. This locus requires an androgen receptor and testosterone for maximum expression. Steroid sulfatase (STS) catalyzes the conversion of steroid sulfates to their active nonconjugated form. In some mammals the steroid sulfatase locus (Sts) is on the Y chromosome, although the rat Sts is on the X chromosome. We measured STS activity levels in SHR and normotensive Wistar Kyoto (WKY) males. SHR had significantly higher STS activity in testes, adrenal gland, liver, and hypothalamus. The Km values for STS in the two strains were not significantly different; thus, activity differences were likely due to differences in enzyme amounts. STS activity was measured in the backcross strains SHR/y and SHR/a to test and/or confirm a Y chromosome influence on STS. STS activity levels in these strains were intermediate between those of SHR and WKY. Because the blood pressures of SHR/y and SHR/a were also intermediate between SHR and WKY, the STS activity could be a secondary response to the hypertension. An alternative hypotheses is that a regulatory locus in addition to the structural locus is responsible for STS activity levels, and this regulatory locus is on the rat Y chromosome. Further study is needed to discriminate between these possibilities, and until the second hypothesis can be eliminated, the Sts locus or its modifier loci remain a potential component of the Y chromosome hypertensive locus.