1. Diets used to reduce sodium intake often involve changes in fats and fibre which might themselves affect blood pressure and/or lipid metabolism. To evaluate the relative importance of these dietary changes for the management of hypertension we have studied the independent and additive effects of sodium restriction (less than 60 mmol/day) and a low fat (30% energy), high P/S ratio (1.0), high fibre (30-50 g/day) 'cholesterol lowering' diet. 2. Ninety-five hypertensives entered a four group parallel study with a factorial design. Following 5 weeks familiarization subjects [BP range 109/66-168/105 mmHg] were randomly assigned to either a 'low sodium, cholesterol lowering' diet or a 'low sodium, cholesterol maintaining' diet. Half the subjects in each group were then assigned to 100 mmol/day NaCl supplement and the remainder to placebo. These diets were continued for 8 weeks. Seventy-nine of the 91 hypertensives who completed the study were on antihypertensive therapy throughout. 3. Mean urinary sodium excretion decreased from 137 (54 mmol/day (n = 43) at baseline (B) to 52 (32) mmol/day (n = 45, P = 0.0001) during intervention (I) in the low sodium groups and remained unchanged in the groups which received slow sodium (B = 129 [46], n = 43; I = 134 [29], n = 42). Diet record and plasma fatty acid analysis confirmed that the dietary aims of the study were achieved. 4. Sodium restriction reduced supine and standing systolic BP by a mean (+/- s.e.m.) of 6 +/- 2 and 6 +/- 4 mmHg, respectively (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)