Pursuing fully a suggestion from linkage analysis that there might be a quantitative trait locus (QTL) for blood pressure (BP) in a Chromosome (Chr) 2 region of the Dahl salt-sensitive rat (DSS), four congenic strains were made by replacing various fragments of DSS Chr 2 with those of Lewis (LEW). Consequently, a BP QTL was localized to a segment of around 3 centiMorgan (cM) or near 3 megabases (Mbs) on Chr 2 by comparative congenics. The BPaugmenting alleles of this QTL originated from the LEW rat, a normotensive strain compared to DSS. The dissection of a QTL with such a paradoxical effect illustrated the power of congenics in unearthing a gene hidden in the context of the whole animal system presumably by interactions with other genes. The locus for the angiotensin II receptor AT1B (Agtr1b) is not supported as a candidate gene for the QTL because a congenic strain harboring it did not have an effect on BP. There are approximately 19 known and unknown genes present in the QTLinterval. Among them, no standout candidate genes are reputed to affect BP. Thus the QTL will likely represent a novel gene for BP regulation.