Arachidonic acid which is precursor of prostaglandins, when administered ($100.0;{mu}g/kg$, or $100.0;{mu}g/kg/min$) intravenously, did not influence on renal function of dog. Arachidonic acid, when infused ($10.0;{mu}g/kg/min$) into a renal artery, produced marked diuretic action accompanied with augmentation of renal plasma flow and with little changed glomerular filtration rate, and exhibited the increased clearances of osmolar substance and free water, and the decreased reabsorption rates of sodium and potassium in renal tubules in only experimental kidney, but did not influenced at all in control kindey. The diuretic acition of arachidonic acid infused into a renal artery was not affected by pretreatment of indomethacin (10.0 mg/kg. i.v) which is inhibitor of cyclooxygenase. Above results suggest that arachidonic acid infused into a renal artery produced diuretic action through direct renal hemodynamic changes, that is mediated by reduction of postglomerular resistance being caused by dilation of vas efferense.