Phase I of the Trials of Hypertension Prevention (TOHP) was a National Heart, Lung, and Blood Institute-sponsored, 3-year, national, multicenter, randomized, controlled trial designed to test the feasibility and efficacy of three life-style (weight loss, sodium restriction, and stress management) and four nutrition supplement (calcium, magnesium, fish oil, and potassium) interventions aimed at lowering diastolic blood pressure in those whose blood pressure was initially in the high normal range (80 to 89 mm Hg). A total of 2182 volunteers were recruited and allocated to the various treatment arms, such that each hypothesis was tested with a power of 85% or higher to detect a diastolic blood pressure treatment effect of 2 mm Hg. The four nutrition supplement interventions were delivered in a double-blinded fashion and the three life-style interventions, single (observed) -blinded. Phase I was designed to provide a rigorous test of short-term lowering of blood pressure for each of the seven treatments chosen and provides the basis for planning of a subsequent long-term trial of hypertension prevention.