We report a case of a renin-producing juxtaglomerular cell tumor and the effects of angiotensin-converting enzyme (ACE) inhibitor captopril or angiotensin II receptor antagonist TCV-116. A 19-year-old female visited our clinic because of hypertension (162/122 mmHg). Plasma renin activity (PRA) was 25 ng/ml/h and plasma aldosterone concentration (PAC) was 880 pg/ml. Abdominal sonography revealed a low echoic tumor in the center of the right kidney. The mass was shown to have isodensity on a computed tomography scan, iso- or slightly lower intensity on T1-weighted magnetic resonance imaging (MRI), and low intensity on T2-weighted MRI. Renal angiography disclosed a hypovascular tumor. Plasma renin activity was not decreased in response to oral administration of captopril (50 mg) or TCV-116 (4 mg), which is associated with a marked decrease in PAC and blood pressure. The average of the mean blood pressure determined by an ambulatory monitoring system every hour for 24 h was lowered from 118 +/- 10.6 (SD) to 85 +/- 11.4 mmHg by TCV-116. Plasma renin activity did not show any difference between the right and left renal vein. Removal of the tumor successfully decreased PRA to 0.3 ng/ml/h and PAC to 33 pg/ml, resulting in a decrease in 24-h average of the mean blood pressure to 78 +/- 16.9 mmHg. The tumor was well capsulated and stained with an antibody against mouse renin. In the tumor, but not normal tissues, renin mRNA was detected at 1.6 kb using rat renin cDNA. The content of active renin in the tumor amounted to 182 micrograms/g tissue, which is 109 times higher than that in normal tissue. The plasma- or tumor-inactive renin concentration was 3.6- or 1.8-fold the active renin concentration, respectively. These results suggest that the juxtaglomerular tumor had an MRI image of iso- to low intensity on T1 and low intensity on T2, and that renin secretion from the tumor may be due mainly to the constitutive pathway and angiotensin II receptor antagonist, as well as to ACE inhibitors, and may be very effective for lowering the blood pressure.