Abstract Rats injected intravenously with just sublethal doses of various organophosphorus cholinesterase inhibitors, during a short lasting hexobarbital anaesthesia, underwent a fall in body temperature of 4 to 6‡C in 3 hours, followed by a recovery in 12 to 20 hours. Two anticholinesterases which ionize in aqueous solution were inactive when given i.v. but active when injected into the subarachnoidal space. During hypothermia the body temperature was abnormally dependent on the environmental temperature. If the hexobarbital was omitted some of the animals showed hypothermia followed by hypenhermia which coincided with severe convulsions. Atropine sulphate reduced the fall in core temperature but was unable to prevent it completely. Atropine methyl nitrate was ineffective. Mouse, guinea-pig and rabbit had no hypothermia when injected with the highest dose of anticholinesterase that did not kill them. It is concluded that cholinesterase inhibitors cause the thermoregulation in the rat to collapse for about 6 hours.