In the spontaneously hypertensive rat (SHR), it has been suggested that as few as 1-6 major genes may be determining of increased blood pressure (BP) and that the identification of these genes might shed light on the pathogenesis of essential hypertension in humans. Accordingly, we propose to: 1) develop a genetic linkage map in the rat and 2) use the map to begin searching for quantitative trait loci (QTLs) that contribute to the pathogenesis of hypertension in the SHR. Specifically, in studies in F2 populations derived from the SHR and selected inbred normotensive strains (Brown-Norway, Lewis, and Wistar-Kyoto), and in 37 recombinant inbred (RI) strains derived from the SHR and the inbred normotensive Brown-Norway rat, we will: 1) Develop a genetic linkage map in the rat by analyzing the inheritance of multiple polymorphic DNA markers. Over 40 polymorphic markers have already been developed and tested in the F2 or RI populations. Additional markers will be identified using single strand conformation polymorphism (SSCP) analysis of specific rat gene sequences, PCR analysis of microsatellite ("CA" type) repeats associated with specific rat gene sequences, and Southern blot analysis of RFLPs associated with single-locus and multi-locus tandem repeat elements marking anonymous gene sequences. The DNA markers will be localized to individual rat chromosomes by somatic-cell hybrid analysis and by linking to other markers with known chromosomal locations. Standard computer programs (MAPMAKER and LINKAGE) will be used to perform the linkage analysis and construct the map. The initial goal will be to have markers spaced at 50 Cm intervals and eventually at 20-30 Cm intervals for use in QTL likelihood mapping. 2) Obtain computerized measurements of arterial pressure in the unanesthetized, unrestrained state. 3) Search for intervals that contain genes regulating blood pressure by constructing QTL likelihood maps (using the MAPMAKER-QTL program to analyze the BP and genetic marker data). 4) Begin to isolate QTLs regulating blood pressure by creating congenic strains that are genetically identical except at the target QTL and a short length of associated chromosome. Such lines could then be used to further localize the QTLs, study their biochemical and physiological effects, and lead to the eventual cloning of specific genes determining of increased BP.