1. The upper thermal death-point of the slow worm (Anguis fragilis) is significantly above that of either the toad (Bufo bufo) or the frog (Rana temporaria). That of the toad differs only slightly from that of the frog.
2. In response to external variation with respect to temperature and humidity, the body temperature of the toad behaves in a manner similar to that of the frog, being considerably below that of the air at low humidities, though appreciably above that of the wet-bulb thermometer. It is concluded that the toad loses water as freely as the frog.
3. The body temperature of the slow worm and that of the alligator (A. mississippiensis) differs only slightly from that of the air under all conditions of sustained air temperature and humidity.
4. The body temperature reaction of the alligator is more sluggish than that of the slow worm, presumably owing to the higher thermal capacity of the animal.
5. The significance of these results in relation to the evolution of the terrestrial vertebrates is discussed. It is suggested that homoeothermic stocks could arise only from reptiles of relatively small size, and that large body size would militate against survival during periods of low mean external temperature.