Calf-fed heifers (n = 72) and steers (n = 72) were supplementedwith 4 levels (0, 0.25, 0.50, and 0.75%) of dietary Mg as MgOduring the final 14 d of finishing and were commingled 1 d beforeslaughter (inducing stress by mixing unfamiliar cattle) to examinethe effects of sex class and Mg supplementation on stress responsesand LM quality characteristics. Heifers and steers exhibitedmarkedly different stress responses. Heifers were more excitablethan steers during preslaughter handling events and exhibiteda short-term physiological stress response that involved acutesympatho-adrenal activation and resulted in increased meat toughness,without a concomitant increase in muscle pH. Steers, on theother hand, exhibited greater physical activity, associatedwith agonistic behavior, during the mixing period and thereforeproduced carcasses with lesser (P = 0.008) LM glycogen concentrationsand greater (P = 0.042) 48-h LM pH values, compared with heifers.Steers also produced tougher (P = 0.008) LM steaks than didheifers. Within the range of pH values observed in this study(5.3 to 6.1), positive, linear relationships between 48-h LMpH and mean LM shear force (P < 0.05) were observed in bothheifers (r = 0.25) and steers (r = 0.37). Effects of pH on LMshear force (P < 0.05), which were most pronounced at 3 and7 d postmortem, diminished during postmortem aging and wereno longer evident (P > 0.05) once LM samples had been agedfor 21 d. Results suggested that toughness of LM steaks frombeef carcasses with final LM pH values greater than 5.65 couldbe problematic unless LM cuts are aged for approximately 18d or longer. Supplementation with dietary Mg increased (P =0.011) serum Mg concentration, but had no effect (P > 0.05)on any of the physiological stress indicators or LM qualitycharacteristics measured in this study. There was no evidenceto support the premise that Mg supplementation of cattle lessensthe effects of preslaughter stress on beef quality characteristics.