Two dynamic tests (Gn-RHi.v. and clomiphene citrate-CC p.o.) were used to evaluate the hypothalamic-pituitary axis in hemodialysis patients and renal transplant recipients (recipients). In the Gn-RHtest the gonadotropin secretion was maximally decelerated in hemodialysis patients while it was normal in recipients. During the CC test a decrease of gonadotropin secretion, chronically and quantitatively identical for both group, was found; while on the following test days an increase was noted, which was more accelerated in male recipients. In cases with uremia a strong negative feedback dominates at the pituitary level probably owing to testicular inhibin. The estrogenic feedback in uremia was intact, while the antiestrogenic feedback at the level of hypothalamus is partly impaired, owing to altered opioid metabolism.