In 24 patients with a functioning kidney transplant and in 33 healthy subjects plasma renin activity (PRA), plasma aldosterone and vasopressin (AVP) levels were assessed under basal conditions and after dietary sodium restriction. Twelve patients were treated with azathioprine and another 12 with cyclosporine A. From the results obtained it seems that immunosuppressive drugs used in transplanted patients ar involved in the pathogenesis of endocrine abnormalities and that the kind of immunosuppressive therapy is of minor importance in their development of maintenance. Transplanted kidneys show a preserved reaction pattern of renin secretion to sodium deprivation and upright position.