1. The effect of a hypotensive dose of intravenous prazosin (0.2 mg/kg) on heart rate and plasma renin activity was evaluated in anaesthetized mongrel dogs pretreated with indomethacin. 2. The effect of prazosin on the renin release elicited by the beta-adrenoceptor agonist isoprenaline and by prostaglandin I2 was also evaluated. 3. Prazosin administration was associated with a significant increase in heart rate and increase in plasma renin activity. 4. Prazosin did not interfere with the increase in plasma renin activity in response to either isoprenaline or prostaglandin I2. 5. We conclude that prazosin is not a unique peripheral vasodilator since hypotensive doses are associated with an increase in heart rate and plasma renin activity. In addition, prazosin does not inhibit the release of renin induced by either isoprenaline or prostaglandin I2.