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AFLATOXIN B1, a carcinogenic metabolite of the fungus Aspergillus flavus, is acutely toxic to a number of species, including rats. Male rats (LD50 7-2 mg./kg.) are more susceptible to the acute oral effects of aflatoxin than are females (LD50 17.9 mg./kg.) (Butler, 1964). Aflatoxin B1 is metabolised to aflatoxin M1 which is present in the blood and urine of dosed rats (Butler and Clifford, 1965) and in the milk of lactating rats, sheep and cows ingesting aflatoxin (de Jongh et al., 1964a; Nabney et al., 1967). Aflatoxin M1 is the hydroxylated derivative of aflatoxin B1 (Holzapfel et al., 1966) and has the same acute toxicity as aflatoxin B1 in ducklings (Purchase, 1967) The presence of aflatoxin M1 in the blood indicates that this compound is a metabolic product of aflatoxin B1. The difference in the susceptibility of male and female rats to aflatoxin may be a result of a difference in the ability of males and females to metabolise aflatoxin. This report describes certain quantitative aspects of aflatoxin metabolism in rats.