Oxygen capacity of reptile blood in vitro is temperature sensitive and is reversibly reduced as much as 40% at the extremes of the temperature range that animals normally encouter. In several species of lizards and snakes, blood oxygen capacity is maximum within the species' activity-temperature range. The correlation coefficient between eccritic temperature and the temperature giving maximum blood oxygen capacity for all species tested was .79 (P < .01). (When the genera Sceloporus and Uta, which show a different pattern, are omitted from the analysis, the correlation coefficient for the remaining species rises to .92.) This adaptation of the blood is one of the mechanisms that produces maximum oxygen consumption near a species' eccritic temperature. In lizards of the genera Sceloporus and Uta and in turtles the maximum blood oxygen capacity occurs at the bottom of the activity-temperature range and may reflect adaptation of these animals to activity at low temperatures.