The effect of an acute intoxication with aflatoxin B1 (AFB1) on some parameters of distal nephron function was examined in rats 48 h after a single i.p. dose of 100 micrograms/kg body wt. The parameters tested were the capacity for the excretion of fixed acids and ammonium salts during metabolic acidosis and the concentration and dilution of urine applying conventional clearance techniques. The treated rats showed a glomerular filtration rate (GFR) approx. 50% lower than the controls, but they were able to reduce the urinary pH as were nonintoxicated animals. The ammonium excretion rate per ml of GFR was unimpaired in the treated rats, but the rate of excretion of fixed acids per ml of GFR was increased. The maximal urinary osmolality was significantly diminished in the intoxicated rats as was water reabsorption, when compared with data obtained in the controls. No differences between groups were seen in the free water formation although urinary excretion of electrolytes was significantly increased. The studies support the nephrotoxicity of AFB1 in the rat probably by interfering with transport function in the collecting tubule cells together with a diffuse impairment of proximal tubule function, as observed previously.