We carried out a blind highly controlled study to investigate the effects of a sunflower-oil-rich diet and a rapeseed-oil-rich diet on the blood pressure of normotensive subjects. Twenty-nine men and 30 women, average age 30 years (range 18-65) were first fed a baseline diet high in saturated fatty acids (19 E% (percentage of total energy), total fat 36 E%) for 2 weeks. According to the crossover design 30 subjects then received a sunflower oil diet high in polyunsaturated fatty acids (13 E%, total fat 38 E%) followed by a low erucic acid rapeseed oil diet high in monounsaturated fatty acids (16 E%, total fat 38 E%) for 3.5 weeks each. The other 29 subjects had the same diets in reverse order. At the end of the saturated fat period systolic blood pressure was 122.6 +/- 11.5(mean +/- SD) mmHg and diastolic blood pressure 75.4 +/- 7.5 mmHg; during the sunflower oil diet the figures were 119.6 +/- 10.3 and 73.9 +/- 7.4 mmHg, and during the rapeseed oil diet 120.1 +/- 11.2 and 72.6 +/- 6.4 mmHg, respectively. There was a significant difference in diastolic blood pressure only between the two oil diets (P less than 0.01). At the end of a 4 weeks' recovery period the systolic and diastolic blood pressures of the subjects were even lower (118.6 +/- 10.6 and 72.3 +/- 8.3 mmHg, respectively) than during the study. These results suggest that the dietary changes had only minor effects - if any at all - on blood pressure in healthy normotensive subjects.