1 A series of studies were undertaken to assess the effect of oral fenoldopam, a specific DA1 dopamine receptor agonist on blood pressure and renal function in patients with mild essential hypertension. 2 Six patients with essential hypertension were entered into a dose-ranging study and received either placebo, 25, 50 or 100 mg fenoldopam. A significant, dose-related reduction in diastolic blood pressure, and increase in heart rate was demonstrated (both P < 0.05), maximal at 45 min to 1 h. Fenoldopam increased plasma renin activity. 3 In a double-blind study, seven patients received a single dose of fenoldopam 100 mg or placebo. Fenoldopam produced a significant fall in systolic (P < 0.05) and diastolic (P < 0.01) blood pressure and renal vascular resistance (P < 0.01). Urine flow rate (P < 0.05), sodium excretion (P < 0.01), plasma renin activity (P < 0.05) and plasma aldosterone (P < 0.05) increased. 4 Five patients underwent measurement of the above parameters following a single dose of fenoldopam 100 mg with a repeat of these measurements after they had taken fenoldopam 100 mg four times daily for 1 month. The acute response of blood pressure to the single dose appeared unchanged but tachyphylaxis was evident in the responses of heart rate, plasma renin activity and plasma aldosterone.