1. Sympathetic reflexes were activated by carotid occlusion in anaesthetized dogs in which changes in renal perfusion pressure were prevented. This produced a prompt and reversible increase in plasma renin activity.
2. Administration of clonidine decreased plasma renin activity, arterial pressure and heart rate and blocked the renin secretory and blood pressure responses to carotid occlusion.
3. These results support the hypothesis that the suppression of renin secretion by clonidine is a consequence of the decrease in sympathetic activity produced by this drug.