Two groups of obese, normotensive men were put on weight-reducing diets in an outpatient study. The groups were comparable with regard to age, weight, heart rate, blood pressure, energy and salt intake during a four-week control period. In a four-week dieting period, Gp I (n = 12) received an energy-reduced diet (1370 kcal, 5.7 MJ) with 24 energy per cent carbohydrates. Group II (n = 11) had an isocaloric diet (1400 kcal, 5.8 MJ) with 59 energy per cent carbohydrates consisting of mainly mono- and disaccharides. Significant decreases in systolic blood pressure, heart rate, plasma noradrenaline and urinary excretion of noradrenaline were observed in Gp I but not in Gp II. Weight reduction and decrease of urinary sodium output was equal in both groups. No difference in alcohol consumption was recorded. We conclude that in obese normotensive patients a high proportion of mono- and disaccharides counteracts the expected hypotensive response of weight reduction. On the other hand, and judged from the present data, the blood pressure decrease observed in the group on a low carbohydrate diet seems to be secondary to an effect on the sympathetic nervous system.