Renal effects of water loading on urine flow rate (V) and concentration (Uosm) were studied in Wistar rats subjected to minor or extensive surgery, under different kinds and level of anesthesia. In the explanted kidney of chronically prepared rats, under light chloralose anesthesia, 1 h.i.v. hypotonic fluid load increased V of the experimental (left) kidney 11 fold while Uosm decreased from 1181 +/- 142 to 210 +/- 80 mosm kg/H2O. In the acutely exposed kidney, under Inactin anesthesia, V increased 5 fold and Uosm decreased from 785 +/- 170 to 204 +/- 57 mosm/kg H2O. In similar experiments under Nembutal anesthesia V increased 4 fold but the urine did not become hypotonic. After acute surgery under Inactin anesthesia water diuresis can be induced but urine concentration before water loading is impaired. Urine hypotonicity after water loading is not achieved in acute experiments under Nembutal--an unsuitable approach to studies of water diuresis. Thus, a transition from antidiuresis to urine dilution can best be accomplished in the chronic explanted kidney model under light anesthesia and without invasive surgery.