Use of coca leaves among the Quechua Indians of the Peruvian altiplano is considered from the biological and social perspectives. Biologically, coca use seems to reduce the loss of body heat in the cold and to enhance working ability to a small degree. It may also have nutritional value. Socially, coca is well integrated into the economic and social systems, providing functions in both spheres. The equation of coca use with cocaine addiction is also considered and a simple equivalence is rejected.