52 children with renal allograft received captopril during 56 periods of treatment. High blood pressure (BP) was due to renal artery stenosis in 22 patients, to diffuse vascular lesions (mainly due to chronic rejection) in 13, to high-dose corticosteroid treatment in the early phase in 11, and to various or unknown causes in 10 patients. In the last 3 groups (including 34 cases) captopril use did not induce any marked drawback. In the absence of overload, a good control of BP was obtained with a mean dosage of 2.2 mg/kg (0.7----5 mg/kg). A mild, transient renal failure (RF) was observed in 6 patients, who were given diuretics without any need. The 22 patients with renal artery stenosis received captopril during 26 periods of treatment. The mean dosage was 1.6 mg/kg (0.3-4.4). The result was excellent in 12 cases (normal BP without any RF), less good in 8 cases (moderate RF +/- borderline BP) and poor (acute RF) in 6. Sodium depletion, due to diuretics in 9, was present in all 14 cases with RF and in 10 of them captopril could be continued or reintroduced without any reappearance of RF after the correction of salt depletion. We conclude that sodium depletion is the main cause of renal failure in transplanted patients receiving captopril and that avoidance of diuretics largely diminishes the risk of RF.