Thermal and metabolic responses of eight male Peruvian Indians native to an altitude of 4000 to 4500 meters were studied during nightlong exposures to moderate outdoor cold (2° to 50 C ). In addition, four of the subjects were studied during cold exposure while chewing coca leaves, alleged by them to have protective effects during cold exposure. The metabolic rate of the eight subjects was elevated during cold exposure but did not differ significantly from averaged values for 1 7 Caucasian control subjects studied under similar conditions in previous experiments. Differences between the thermal responses of the Indian subjects and of the same Caucasian controls were noted in the generally higher hand and foot temperatures, and the correspondingly lower rectal temperatures, of the high altitude dwellers. The Indian subjects using coca did not display any measurable differences in their responses with or without the drug but, by observation and by their own report, they did sleep longer and more comfortably when using coca.