In an effort to elucidate physiological factors involved in the development of pale-soft-exudative pork, blood acid base status was assessed in swine from two genetic lines of pigs and their F1 cross. The lines consisted of: (1) pigs that reacted positively (skeletal muscle rigidity) to the respiratory administration of halothane (halothane positive (H+)) based on Pietrain × Lacombe breed crosses, (2) Purebred Lacombe pigs that did not react positively to halothane anesthesia (Lac) and (3) pigs which were the progeny of crossbreeding (C) between halothane positive and negative animals. In addition, time off feed prior to slaughter (0, 24 or 48 h) was imposed as a stressor in order to test response differences among the three lines. The venous blood PCO2, total CO2, bicarbonate ion levels, standard bicarbonate and base excess levels were found to be higher in the H + pigs compared to either Lac or C pigs. All pig lines displayed higher blood pH, total CO2, bicarbonate ion, standard bicarbonate and base excess yet lower PO2 at 24 h off feed compared to 0 h off feed. These data suggest that H+ pigs have a greater tendency toward hypercapnia and a blood base excess than either Lac or C pigs. In addition, the incidence of hypercapnia and blood base excess for H +, Lac and C pigs was greatest at 24 h off feed. Key words: Acid-base stability, pig genotypes, fasting