The renin-angiotensin system and renal function in kidney transplantation. The use of converting enzyme inhibitors (CEI) has permitted us to assess the role of the renin-angiotensin system in the control of arterial pressure and renal function in various conditions. In renal transplant recipients treated by azathioprine and steroids, the occurrence of CEI-induced deterioration of renal function is highly suggestive of renal artery stenosis, whereas renal vasodilatation associated with unchanged glomerular filtration rate in response to CEI is indicative of a significant role of native kidneys. In hypertensive recipients without renal artery stenosis, the absence of renal hemodynamic changes after CEI may be predictive of subsequent chronic rejection. The information provided by CEI is rather different in cyclosporine treated subjects. In this setting, no acute effect of CEI on renal hemodynamics is detectable. Whether the renal response to CEI is similar in cyclosporine when compared to conventionally treated patients with renal artery stenosis remains to be demonstrated. 