The disposition of tritiated aflatoxin B1 in the rainbow trout following oral and intravenous administration was studied over a period of 8 days by means of liquid scintillation counting and whole-body autoradiography. The pattern of distribution together with the quantitative measurements were fairly similar in both groups, indicating a high degree of gastrointestinal absorption. The highest concentrations of radioactivity were observed in the bile, liver, kidney, pyloric caeca, uveal tract of the eye and the olfactory rosette. Substantial amounts of radioactivity were still present in the liver, kidney, olfactory rosette and the mucosa of the pyloric caeca 8 days after administration. A major fraction of this radioactivity was not extractable with certain polar and nonpolar solvents, indicating covalently bound metabolites.