A mathematical analysis of continuously proportional and neutral zone (dual threshold) temperature regulation leads to criteria for experimentally distinguishing between these differing controls. Colonic, skin, and head temperatures were measured in a lizard as it shuttled between hot and cold environments. An analysis of body temperature when hot-environment temperature was 65 C shows that the lizard was thermoregulating (P < .05). A further analysis of behavior at a hotenvironment temperature of 47 C shows that the body temperatures initiating exit from the hot and cool environments were significantly (P < .05) different. Furthermore, shuttling ceased when cool-environment temperature was increased to 36 C or hot-environment temperature [...]