1. The effect of clonidine HCl on basal and isoprenaline-stimulated renin secretion was examined in the isolated rat kidney. 2. Intrarenal infusion of clonidine at a dose level which did not increase renal perfusion pressure (1.9 nmol min-1 g-1), significantly lowered basal renin secretion without altering the stimulatory response to isoprenaline. 3. Increasing the dose of clonidine (3.4 nmol min-1 g-1) raised renal perfusion pressure, lowered perfusate flow rate and attenuated the response in renin secretion to isoprenaline. 4. Since the vasoconstrictor effect of clonidine is considered to be due to alpha-adrenergic stimulation, this observation is similar to a previous study reporting suppression of renin secretion by vasoconstrictor infusions of catecholamines (Vandongen & Peart, 1974). 5. These studies demonstrate a direct intrarenal inhibitory effect of clonidine on renin secretion. It is suggested that this may contribute to the suppression of plasma renin levels observed in vivo following administration of this drug.