1. In this study an acute anti-hypertensive effect of three anti-oxidant agents (vitamin C, thiopronine and glutathione) in hypertensive subjects and in both hypertensive and non-hypertensive diabetic patients is reported. 2. The anti-oxidants had no effect on blood pressure in healthy normal subjects at a dose of 6 mmol, but thiopronine and glutathione produced a significant hypotensive effect at a dose of 12 mmol. 3. These data suggest that anti-oxidants might have a dilatatory effect and that an imbalance of the nitric oxide-free radical interaction might facilitate the development of hypertension in humans.