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Hypertension is a complex disease in which environment and heredity interact. It is a polygenic trait that can be studied by investigating its intermediate phenotypes. We have selected the population from the Saguenay-Lac St. Jean region because it is a relative genetic isolate. We have found that the prevalence of hypertension at every age level was greater than in a control Canadian population. A sib pair analysis was used to measure familial correlation and heritability of more than 200 phenotypes among which were renal functions, anthropometric measures and cardiac morphology and reactivity. Very strong familial correlations were estimated for several cardiovascular phenotypes. During a pharmacological infusion of norepinephrine, we observed that systolic blood pressure was increased as expected but that the heritable component of this phenotype increased from null at baseline to close to 70%. Preliminary total genome scans completed on the DNA of the participants have selected several regions linked to these phenotypes. This type of study opens the door to pharmacogenetic studies in which sib pairs are analyzed for their concordant or discordant response to a medication.