To examine the medullary washout theory of pressure diuresis, anesthetized diabetes insipidus dogs were subjected to variations in renal arterial pressure in acute experiments. Changes in arterial pressure within the range of renal blood flow autoregulation caused significant and proportionately greater increases in urine flow. There was a significant relationship between arterial pressure and GFR although autoregulation was observed. A significant relationship also existed between the changes in GFR and the changes in urine flow such that the urine flow changes could be accounted for by the GFR changes. The changes in urine osmolality exhibited a negative regression on arterial pressure, but were quantitatively very minimal. The results demonstrate that the pressure diuresis phenomenon can occur from factors other than medullary washout.