This thesis is an investigation of the stress response of salmonid fish and the regulation of the endocrine mechanisms of growth during changes in physiological conditions. Controlled by the HPI axis, the stress response incurs extensive catabolic demand on endogenous metabolite stores at the expense of growth through catabolic actions under the assumed direction of cortisol. It is suspected that the stress response also suppresses the growth-promoting actions of the GH/IGF-I/IGFBP axis. The central theme of this thesis was to characterize the influence the stress response on the endocrine regulation of growth during conventional (e.g. emersion, salinity transfer) and unconventional stresses (e.g. competition, social interaction, parasite infection), using rainbow trout (Oncorhynchus mykiss) and Chinook salmon (Oncorhynchus tshawytscha) as models. Findings corroborate the inhibitive impact of chronic stress on growth through catabolism of endogenous metabolites in the presence of cortisol levels representative of moderate stress in salmonids. Trout in infected with Cryptobia salmositica, demonstrated similar evidence of pathogen-induced growth suppression via changes in catabolic elements within the GH/IGF-I/IGFBP axis in a similar manner to hypercorticoidic fish, but without elevated cortisol despite clear physical duress. Accompanying reduction in food intake and change to nutritional status influenced much of the growth-suppressing impacts observed on the endocrine axis during disease incidence. Moreover, Cryptobia infection inhibited cortisol signaling and production the pituitary and in the interrenals, respectively. In Chinook salmon, the endocrine response to stress was altered by parental breeding strategy and early rearing environment; traditional hatchery breeding and rearing methods impacted growth performance during physiological challenge when contrasted to mate choice cohorts. Rearing Chinook in a semi-natural channel environment revealed clear differences in performance between these fish stocks that were not observable in the hatchery environment. Moreover, social interaction and competition between mate choice and hatchery-bred salmon influenced the inhibitive effects of the stress response on growth performance, as well as the physiological responses to endocrine-regulated changes during smoltification. This thesis characterizes the novel regulatory actions of the stress response on the endocrine growth axis via regulation of both central and peripheral elements of the GH/IGF-I/IGFBP axis. / NSERC, OGS