The role of intratubular obstruction in the pathogenesis of folic acid-induced renal failure was studied. During 3 h following injection of 250 mg/kg i. v. of folic acid in the rat, urine flow was diminished (less than 1 µl/min/100 g), inulin clearance fell below Vioth of the control values, lissamine green passage time through proximal surface tubules was prolonged up to 65 sec and intratubular pressure rose to 40 mm Hg or higher. Folic acid casts could be seen on the cryostat sections in the lumen of the tubules and in the urine. Acute renal failure could be prevented either by massive furosemide diuresis or by alkahnization with small amounts of intravenous NaHCO₃. Split oil droplet experiments showed a reduction of the proximal tubular reabsorptive capacity at 2 and at 24 h after folic acid injection. However, a similar impairment was measured after bilateral ureteral ligature at 2 and at 24 h without folic acid. It is concluded that folic acid, because of its poor solubility particularly at an acid pH, precipitates intratubularly as a consequence of fluid reabsorption and increasing acidity along the nephron. Intratubular obstruction due to folic acid crystals thus appears to be the primary cause of acute renal failure induced by folic acid injection. Reduction of proximal tubular reabsorptive capacity is probably due to intratubular obstruction rather than to a direct effect of folic acid on tubular enzyme activity described earlier.