The present study was performed to determine if pigs varying in their susceptibility to stress, adapted to a casein-based diet, experience an improvement in biochemical parameters related to stress and behaviour compared to pigs adapted to the traditional western canadian cereal-based swine diet. Experiment 1 involved separating fifty-eight, 8-week old pigs according to genotype with respect to the halothane gene. Within each genotype pigs were divided into 2 groups and assigned to either a control diet or to a diet where most of the protein source was substituted for casein. All animals were adapted to diet for 6 weeks and experienced a weekly blood sampling stressor. Day 1, 14 and 35 of the plasma samples were analyzed for glucose, cortisol, ACTH, insulin, pyridoxal 5$ sp prime$-phosphate (PLP), amino acid concentrations and dopamine-$ beta$-hydroxylase (DBH) activity; metabolic indices known to be responsive to stress. Experiment 2 involved separating fifty-seven, 14-week old pigs in the same manner, then adapting the pigs to their respective diets for a period of 4 weeks. Afterwards, pigs were transferred from their pen to a novel pen-maze situation where they had their behaviour monitored for a period of one hour. Results of these experiments have revealed that (1) the stress susceptible and carrier pigs experienced reduced day 35 plasma glucose, PLP concentrations and DBH activity compared to normal pigs; (2) dietary adaptation to the casein diet resulted in greater day 14 and 35 PLP levels and day 35 essential amino acid lysine, threonine, methionine, tryptophan and arginine concentrations compared to control-adapted pigs; (3) the carrier pigs investigated their surroundings more frequently than the stress susceptible pigs, and the normal pigs engaged in the through-maze behaviour more often than the stress susceptible pigs; and (4) adaptation to the casein diet, compared to the control diet, resulted in fewer displacement-type behaviours such as drinking

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