The present study reports the hormone profile [growth hormone (GH) and thyroid hormones] in a fast- and slow-growing strain of rainbow trout under different feeding regimes. An examination as to whether growth rate could be related to the endogenous levels of these growth-promoter hormones was also undertaken. Fish were fed to satiation to establish routine feeding behaviour; others were submitted to a restricted ration level (2% of body weight) to nullify the feed intake effect, thereby allowing a comparison of the general metabolism of the fish, over a period of 16 weeks. Finally, fish were submitted to a 30-day fasting period, which functioned as a test to amplify the capacity of the animals to maintain high levels of circulating hormones. Differences in growth rate between the two rainbow trout strains, when fed to satiation, could not be associated with differences in hormonal status (pituitary and plasma GH, plasma T3 and T4). Nonetheless, fish of the fast-growing strain exhibited significantly higher circulating GH values than slow-growing fish, after the fasting period. Additional studies on both the thyroid hormones and GH receptor systems, and the cellular response to these hormones are warranted to further characterize this aspect of the endocrine regulation of body growth in the two rainbow trout strains.