Photosynthetic and respiration rates were measured in the field on plants of many species growing between the elevations of 4,500 and 12,700 ft in the White Mts. of California. No relation was found between photosynthetic rate and the elevation at which the plants were growing. However, plants representing both tree and shrub species of high elevations generally had greater respiration rates and lower photosynthesis/respiration ratios at the same temperature than did members of low elevation species. Gas exchange rates of plants of diverse elevational origin grown from seed in a uniform ernvironment were measured at a series of temperatures and CO2 levels. Certain of the trends and rates measured in the field were not maintained in the greenhouse-cultivated plants, indicating considerable, environmentally conditioned plasticity in gas exchange capacity. However, when cultivated in a uniform environment the high elevation plants generally displayed peak photosynthetic activity at temperatures lower than the low elevation plants. No relation was found between the elevation origin of a plant and photosynthetic efficiency at reduced CO2 levels.