Various drugs have been tested for a capacity to prolong the hypnotic effect of chloral hydrate in mice. Amongst the compounds which, when injected subcutaneously in substantial amount shortly before the chloral hydrate (250 mg./kg. intraperitoneally), increased sleeping time significantly were adrenaline, noradrenaline, phenylephrine, methoxamine, 5-hydroxytryptamine (serotonin), histamine, ergotamine, yohimbine, and atropine. The ability of these drugs to prolong chloral hydrate sleeping time could not be related to any common circulatory property, but most of the active drugs are known to lower body temperature under comparable conditions. It was found that mice which have been pre-treated with 5-hydroxytryptamine or adrenaline suffer a much greater fall of body temperature when chloral hydrate is given subsequently than do mice which have been given chloral hydrate alone. It is suggested that some, at least, of the drugs which prolong the effects of hypnotics do so by virtue of a hypothermic action.