To assess the role of the kidney dopamine system on the diuretic state induced by angiotensin-converting enzyme (ACE) inhibitors, we examined the changes in urinary excretion and plasma level of dopamine, and kidney dopamine receptors in spontaneously hypertensive rats (SHR) treated with cilazapril, an ACE inhibitor. We administered cilazapril 10 mg/kg orally to 13-week-old SHR daily for 21 days (CILAZA group). Systolic blood pressure was significantly decreased in the CILAZA group on Day 6 compared with that in vehicle-treated SHR (control group). The urine volume was three- to fivefold higher in the CILAZA group, and total urinary dopamine secretion was also increased compared with the control group. There was no significant difference in affinity and number of kidney dopamine receptors between the CILAZA and the control groups. In conclusion, the diuretic effect caused by cilazapril is partly mediated by inhibition of the water reabsorption via the increase of dopamine production in the kidney.