1. Cholera enterotoxin was used to evaluate a possible role of endogenous cyclic AMP in production of hyperthermia. Injection of purified toxin (0.10-1.0 mug in 0.10 ml.) into the lateral cerebral ventricle of unanaesthetized cats caused dose-related hyperthermic responses. Heating the toxin for 40 min at 90 degrees C abolished its hyperthermic activity.2. Intraventricular administration of dibutyryl cyclic AMP (250-1000 mug) also caused hyperthermia which, however, was preceded by transient periods of hypothermia and/or excitation in about half of the tests.3. Paracetamol, indomethacin and sodium salicylate inhibited hyperthermic responses to cholera enterotoxin. Paracetamol and indomethacin also inhibited hyperthermia induced by dibutyryl cyclic AMP (sodium salicylate was not tested).4. It is likely that the hyperthermic effect of cholera enterotoxin in the cat is mediated via endogenous cyclic AMP and that the antipyretics inhibit this effect by an action subsequent to the increase in cyclic AMP.5. It is unlikely that prostaglandin-induced hyperthermia in the cat is mediated via cyclic AMP since these antipyretics do not inhibit this response to prostaglandin E(1).