Fifteen patients (eight men, seven women) with hypertension and renal artery stenosis underwent dilation of the stenosis by percutaneous transluminal renal angioplasty (PTRA). During and shortly after this treatment the effects on the renin-angiotensin-aldosterone system and blood pressure were studied. Plasma renin activity (PRA) was measured in peripheral blood and in renal venous blood during the PTRA. PRA increased in peripheral blood during PTRA as a result of an immediate significant rise in renal venous plasma renin activity by 132 +/- 134% (P less than 0.01) on the dilated side. PRA in the contralateral renal vein was close to that in peripheral blood. Within 10 min after PTRA there was a significant increase in serum aldosterone from 439 +/- 343 to 774 +/- 635 pmol 1-1 (P less than 0.025), while serum cortisol remained unchanged. The aldosterone increase was most probably mediated by angiotensin II. Systolic and diastolic blood pressures were unchanged during PTRA in spite of renin and aldosterone increases, suggesting that antihypertensive factors counteract the pressor effects of a physiologically relevant increase in PRA.