Tests were made with histamine diphosphate and the following antihistamines: mepyramine-HCl, diphenhydramine-HCl, tripelennamine-HCl, promethazine-HCl (phenergan), phenindamine tartrate (thephorin), prophenpyridamine maleate, thonzylamine-HCl, 10-[2-(1-pyrrolidinyl)ethyl]phenothiazine-HCl (pyrathiazine-HCl), antazoline-HCl, thenylpyramine-HCl (methapyrilene), chlorocyclizine-HCl, 1-cyclohexylhexahydrobenzoic acid 2-diethylaminoethyl ester-HCl (dicyclomine-HCl), RP 3015, [2-(10-phenothiazinyl)isopropyl]trimethylammonium benzenesulfonate (thiazinamium), and RP 3550.Histamine and antihistamines cause a decrease in the body temperature of mice, rats, and guinea pigs, but they have no effect on the body temperature of rabbits.Antihistamines reduce the body temperature of normal animals as well as those with induced fever; therefore, the effect cannot be attributed to true antipyretic activity.There is no parallelism between the body temperature-decreasing effect of the antihistamines and their antihistaminic potency, antianaphylactic activity, atropine-like effect, hypnotic action, and acute toxicity.