Acute tannic acid toxicity was studied in 6 adult Merino ewes by administering 7-10% (w/v) tannic acid solution once into the abomasum via an abomasal fistula at dose rates of 0.5, 1.0 or 2.0 g tannic acid/kg body weight. Samples of blood, urine and abomasal fluid were collected over a 48-h period before the sheep were necropsied 48 h after dosing. Dose rates of 1.0 and 2.0 g tannic acid/kg body weight produced abomasal, liver and kidney lesions. The abomasum had superficial mucosal erosions, hemorrhage and submucosal edema; the liver showed midzonal or periacinar coagulative necrosis; and the kidneys had focal tubular vacuolation and necrosis. In sheep dosed with 1.0 g tannic acid/kg body weight, there was a significant (P < 0.01) decrease in plasma phosphate and an increase in plasma aspartate aminotransferase and urea levels. Plasma total protein concentrations significantly (P < 0.05) decreased 24 h after dosing, but returned to normal by 48 h. By 4 h after dosing, abomasal fluid pH was significantly (P < 0.01) higher than predosing value, whereas urine pH significantly (P < 0.05) decreased. Both abomasal fluid and urine pH returned to normal 48 h after dosing. The liver and kidney damage seen in this study, in contrast to the methemoglobinemia without liver and kidney lesions previously reported when sheep were given tannic acid by oral gavage (1), suggest that, in cases of natural poisoning, tannins are not released from plant material until it enters the abomasum.