The spontaneously hypertensive rat is the most widely studied animal model of essential hypertension, yet the genetics of transmission of high blood pressure in this strain have not been clearly defined. It has been proposed that in the spontaneously hypertensive rat, blood pressure follows a simple additive mode of inheritance and that the hypertension is primarily determined by a single major locus. To investigate the genetics of transmission of increased blood pressure in the spontaneously hypertensive rat, we performed a biometric genetic analysis of multiple, direct measurements of arterial pressure in unanesthetized, unrestrained rats derived by crossing spontaneously hypertensive rats with two different inbred normotensive strains, the Charles River Wistar-Kyoto rat and the Lewis rat In both crosses, approximately 60% of the variation in blood pressure could be assigned to genotypic variation. The data fit an additive-dominance model of inheritance in which alleles decreasing blood pressure were partially dominant Thus, in offspring derived from crosses between spontaneously hypertensive rats and Wistar-Kyoto rats or spontaneously hypertensive rats and Lewis rats that are raised under ordinary laboratory conditions, increased blood pressure is not determined by simple additive effects of alleles at a single major locus. The current findings are consistent with the possibility that in the spontaneously hypertensive rat, hypertension may arise from mutations in alleles that ordinarily act in a dominant fashion to suppress blood pressure.