We used a cross between the stroke-prone spontaneously hypertensive rat (SHRSP) strain and the Wistar-Kyoto (WKY) normotensive strain to elucidate the genetic basis of hypertension. Previous studies have reported conflicting evidence for the contribution of the Y chromosome to hypertension in these models. To investigate further the role of the Y chromosome in hypertension, we performed two large reciprocal crosses: one with the SHRSP as a male progenitor of the cross, yielding 60 F2 rats, and another with the WKY as a male progenitor, yielding 83 F2 rats. The resulting F2 hybrids were phenotyped with the use of a radiotelemetry system (Data Sciences) for measurement of systolic, diastolic, and mean arterial pressures as well as heart rate and motor activity continuously for 96 hours at baseline and after 1% NaCl was added to the rats' drinking water for 12 days. Male F2 hybrids with the SHRSP grandfather had significantly higher average systolic, diastolic, and mean arterial pressures at baseline compared with male F2 hybrids with the WKY grandfather (188.7 +/- 18.1 versus 168.9 +/- 11.5, 130.3 +/- 14 versus 115.7 +/- 7.3, and 159.1 +/- 15.8 versus 141.5 +/- 9.4 mm Hg, respectively). These differences were also observed after salt loading (197.9 +/- 22.1 versus 176.8 +/- 11.7, 136.5 +/- 17.3 versus 120.7 +/- 7.6, and 166.7 +/- 19.5 versus 148 +/- 9.7 mm Hg, respectively; P < .0001 for each comparison). These results suggest that the SHRSP Y chromosome contains a locus or loci that contribute to hypertension in SHRSP/WKY F2 hybrids.