Two 600-kg lactating cows were each given a single oral dose (0.5 mg/kg of body weight) of aflatoxin B1 (B1). Samples were obtained at postdosing hours 0, 1, 2, 3, 4, 6, 8, 10, and 12 and thereafter every 12 hours for 10 days. Aflatoxicol (Ro), B1, and aflatoxin M1 (M1) were found in the milk, plasma, and RBC of both cows at postdosing hour 1. Maximum concentrations of the toxins were observed at 12 and 60 hours. The ratio of the concentrations for Ro, B1, and M1 was approximately 1:10:100. Both cows had clinical signs of distress at 24 hours; 1 cow died at 60 hours and the other cow recovered within 4 days. In the samples of liver, kidney, urine, bile, and rumen contents of the cow that died, the B1 concentrations were 5.1, 3.3, 4.1, 1.6, and 320 ng/g, respectively, and the M1 concentrations were 4.3, 20, 37, 16, and 8.6 ng/g. The Ro concentrations in the kidney were approximately equal to that of B1; however, liver, urine, bile, and rumen contents concentrations were 0.88, 0.10, 0.36, and 4.9 ng/g, respectively.