A study was performed on seven patients with Cushing's disease and seven age and sex matched patients with essential hypertension. The patients maintained their usual diet and sodium intake and received the ACE inhibitor trandolapril at a dose of 2 mg daily for 8 days. Trandolapril treatment resulted in an increase in the active renin and decrease in the urinary kallikrein activity in both study groups. Blood pressure decreased significantly in patients with Cushing's disease and essential hypertension after 8 days of trandolapril treatment. The opposite responses of renin and kallikrein to ACE inhibition support a possible feed-back mechanism of regulation between the renin-angiotensin system and the kallikrein-kinin system.