It has been generally assumed that normally endothermic mammals have hemoglobins with greater temperature sensitivity than ectotherms or hibernating mammals. We found the pig to be an exception to this rule. We measured 101 dissociation curves using biotonometry on fresh heparinized blood from 9 pigs at 4 temperatures. The partial pressures of O2 at 50% saturation (P50 +/- SE) were 27.8 +/- 1.2, 30.0 +/- 1.3, 35.7 +/- 0.6 and 41.6 +/- 1.8 mm Hg at 30, 33, 37, and 41 degrees C, respectively. The temperature coefficient d log P50/dT was 0.016 +/- 0.002, about two-thirds that of human and dog blood. It was also saturation dependent, being significantly greater at lower saturations than at high saturations. This saturation dependence causes an increase in heme-heme cooperativity in binding oxygen at higher temperatures. The fixed acid Bohr coefficient was -0.441 +/- 0.005 at 37 degrees C and was not temperature sensitive. We conclude that the effect of temperature on the porcine dissociation curve is significantly lower than that reported for other endotherms, and is similar to that previously reported for hibernating mammals and some ectotherms.