We studied factors affecting the ability of common brushtail possums (Trichosurus vulpecula) to regulate their intake of a dietary toxin, jensenone, extracted from Eucalyptus leaves. Increasing concentrations of jensenone in the diet led to a dose-dependent decrease in food intake best described as an exponential decay. Animals that had not previously been exposed to jensenone ate significantly more when first offered food containing the compound than on subsequent days. However, when offered the same amount of food in a number of portions throughout the night, naive animals ate significantly less than animals offered the total meal at once. When offered food containing jensenone over a 13-day period, the animals' intake varied cyclically with relatively high food intakes followed by relatively low intakes. Furthermore, animals that were exposed to cold conditions (4 degrees C) ate more than those maintained at 18 degrees C but this difference was abolished when jensenone was included in the diet. We interpret these results as showing that regulation of toxin intake by common brushtail possums depends on learned responses that can override other important influences on feeding.