1. Two experiments with mal: weanling rats were conducted in which they received individually and restrictedly either a basal semi-purified diet containing starch as the principal carbohydrate or the same diet to which mixed aflatoxins were added in quantities providing from 0-1 3 to 0.4mg aflatoxin BJkg diet. Various natural ingredients, or semi-purified sources of dietiry fibre were substituted for a portion of the starch in the basal diet containing aflatoxin. The diets were fed fcr 13-14

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... ks after which the rats were given ad lib. a commercial rodent diet until they were killed at 109 weeks of age. 2. Two further experiments were conducted in which twenty-four rats in each experiment received the basal diet plus aflatoxin, or diets in which a portion of the starch was replaced by gum arabic or by wheat offal. After 13 or more weeks the absorption, retention and excretion of W-labelled aflatoxin B, was measured in each rat. 3. The addition of gum aratdc or wheat bran to the diet decreased the effects of the toxin in the first two experiments, but as measured b) several characteristics, only wheat bran provided an effect which persisted during the period when neither it nor the toxin was given. The effects included an apparent reduction in tumour incidence. The change in the content of starch in the basal diet, occurring as a consequence of adding the test ingredients is also considered to be an associated cause of the effects observed. 4. In comparison to starch, wheat offal increased the total 14C in the faeces and the proportion of the total found during the first 48 h after dosing. Rats receiving starch excreted more 14C in their urine and retained more 14C in their livers. The differences between gum arabic and starch were not significant as measured by 14C excretion and retention. Liver size as a proportion of carcass weight was less in rats receiving wheat offal or gum arabic, and rats receiving wheat offal had a lower incidence of fat-loaded hepatocyks. 5. The interaction of dietaq toxins, or drugs, with the ingredient composition of diet may affect animal response even when the diets arc adequate and similar in nutrient composition. This may be of significance both in drug-safety studies and in animal production.