Graded doses of pure ochratoxin A (0, 0.5, 1.0, 2.0, 4.0, and 8.0 μg of toxin per g of feed) were incorporated into a commercial diet which was fed to chicks from 1 day to 3 weeks of age, at which time the experiments were terminated. Growth was inhibited at 2.0, 4.0, and 8.0 μg/g, whereas the kidneys were enlarged at doses of 1.0 μg/g and above. Renal function as measured by clearance of phenol red was decreased 15 and 31% by doses of 4.0 and 8.0 μg/g, respectively. Uric acid was increased 38 and 48% over the control values by doses of 4.0 and 8.0 μg/g, respectively. The plasma electrolytes Na, Cl, Ca, and K were measured; however, only K was significantly (P < 0.05) altered, showing a decrease at doses of 4.0 and 8.0 μg/g. The percentage dry weight of the kidneys decreased significantly at dose levels of 4.0 and 8.0 μg/g, indicative of edema. Histological examination of kidney sections gave the impression of edema and some tubular necrosis. Pathological changes were observed at all dose levels. These data demonstrate that ochratoxin A is a severe nephrotoxin in young broiler chickens.