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This experiment was designed to measure the effects of infestation by B. microplus on cattle and to separate the effects of reduced food intake ("anorectic effect") from those due to the remaining factors of tick infestation ("specific effect"). Hereford cattle kept on a high-quality diet were studied over a treatment period of 11 weeks with the tick-infested animals being infested regularly with equal larval doses for each animal.The anorectic effect accounted for approximately 65% of the depression of body weight due to tick infestation. Body weights were not related to the numbers of maturing female ticks counted on the infested animals. However, the body weights were related to food intake, the large variation in which was considered a reflection of the variable effect of the toxic principle of the tick on the appetite of the cattle.After treatment, tick-infested cattle were kept clear of tick and run with the two control groups of cattle. Pasture was supplemented with extra rations. The compensatory gain made by the infested group was less than that of the group which had been matched with it for food intake and kept tick-free. This indicates a severe effect on the metabolism of the tick-infested animals, with prolonged after-effects.