Phase I was a pilot study to test the feasibility of providing and obtaining compliance with selected nutritional-behavioral, non-pharmacologic interventions and to measure the short-term effectiveness of the interventions on reducing or preventing an increase in diastolic blood pressure. Ten clinical centers, and a coordinating center, which included two central laboratories and a nutrient data center, participated. Each clinic tested a subset of the interventions. The lifestyle arm of the trial, which included weight loss, sodium restriction, and stress management, had an open design with untreated controls. The two-stage supplement arm of the trial, which tested magnesium and calcium in Stage I, and fish oil and potassium in Stage II, was double-blinded and placebo-controlled. Three clinics tested only lifestyle interventions, two tested only supplements, and five participated in both arms of the trial. Recruitment began in August 1987 and randomization was completed by October 1988. Endpoint data collection was completed in January 1990. The average follow-up was approximately 18 months.