In the laboratory, Phrynosoma cornutum and Phrynosoma douglassii from central New Mexico had mean preferred body temperatures of 38.5 C and 35 C respectively.... Differences in the oxygen pulse values of these species at high and low temperature extremes indicate differences in the cardiac output and efficiency. It is suggested that P. douglassii may have a more efficient transport of oxygen by the respiratory pigments at low temperatures than P. cornutum.