Twelve clinically normal Shetland ponies were allocated to one of four treatment groups. Aflatoxin B1 was administered at the dosage level of 2 mg/kg of body weight to group A, 1 mg/kg to group B, and 0.5 mg/kg to group C; a placebo was given to group D (controls). Plasma samples were assayed at 4-hour intervals for iditol dehydrogenase (ID) (sorbitol dehydrogenase) concentrations as an indicator of hepatic damage. One of the ponies in group A died 68 hours after dosing; another pony in group A died 76 hours after dosing. All other animals survived the experiment. The means of peak ID values were as follows: group A, 1514.0 IU/l (P < 0.05); group B, 192.6 IU/L (P < 0.05); group C, 8.5 IU/L (P < 0.05); and group D (controls), 2.7 IU/L. A square root transformation analysis of the postdosing response of ID values in relation to time demonstrated that the mean of group A became significantly higher (P < 0.05) than did the mean of the control group at 5 to 6 hours. The mean of group B at 12 to 16 hours and the mean of group C at 20 to 24 hours also were significantly higher (P < 0.05) than was the mean of the control group.