Twenty-three moderately obese middle-aged men with previously untreated hypertension (World Health Organization classification 1-2) were evaluated to assess the effects on blood pressure (BP) of a diet restricted in energy (fats and carbohydrates) but unrestricted in sodium (Group 1) compared to a diet restricted in energy and sodium (Group 2). The patients were randomly allocated to either of the two groups and were comparable in age, sex, weight, and BP. The same energy- and sodium- restricted diet was given to both groups, but the intake of Group 1 (n = 13) was supplemented with dietary sodium. The average urinary output for Group 1 was 192 +/- 39 mmol/24 hr at baseline and 200 +/- 56 mmol/24 hr during the diet. For Group 2 (n = 10), which remained on the initial diet, urinary sodium excretion changed from 188 +/- 53 mmol/24 hr at baseline to 97 +/- 32 mmol/24 hr (p less than 0.001). Intraarterial BP, cardiac output (CO), plasma volume, circulating norepinephrine (NE), and urinary NE were measured at baseline and at the end of the dieting periods. Before the dietary sodium supplement while on the initial diet, the patients in Group 2 showed a reduction in body weight from 97.3 +/- 10.5 kg to 88.6 +/- 9.9 kg (p less than 0.001). Heart rate (HR) and urinary NE output were significantly reduced in comparison with baseline, but intraarterial BP was unchanged. No change in cardiopulmonary blood volume, CO, or stroke volume (SV) was observed.(ABSTRACT TRUNCATED AT 250 WORDS)