Monk parakeets have been introduced into North America within the past 15 years and are apparently becoming established in several geographical regions. Several physiological responses of monk parakeets related to climatic tolerance were examined, and it is concluded that the species is equipped physiologically to occupy most climatic situations with the exception of arctic and subarctic regions and waterless deserts. The standard metabolic rate, determined during the winter, was 44% lower at night (1.17 ml O2 g-1 hr-1) than during the day (1.68 ml O2 g-1 hr-1). Monk parakeets are relatively tolerant of low air temperature (Ta) and showed no signs of hypothermia at Ta's as low as-8°C. The birds were unable to maintain body weight on a diet of air-dried seeds without supplemental water. Monk parakeets possess excellent capabilities for increasing evaporative water loss at high Ta's, being able to dissipate up to 153% of their metabolic heat production at 44°C. This species responds to high Ta's with open-mouthed panting. During panting the thick, moist tongue is raised and lowered in synchrony with the thorax. Thus, monk parakeets may employ lingual flutter to augment evaporative cooling; a mechanism analogous to the gular flutter of other nonpasserine birds.