Investigation of factors controlling seasonal changes in thyroid activity of juvenile steelhead trout (Salmo gairdneri Richardson) in fresh water revealed positive correlations between temperature and radioiodine assessments of thyroid activity. Temperature and thyroid epithelial height, however, showed negative correlations. These correlations were verified experimentally. In yearlings the thyroid was refractory to increasing photoperiod (January to July) but two-year-old potential migrants

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... wed a positive response at the same season. Thus, the high smolt thyroid activity is induced by the combined influence of rising temperature and increasing spring photon period. Body mass (logarithm) was inversely related to various I¹³¹parameters (logarithm), so that small fish had higher thyroid activities than large fish. Precocious sexual maturation of two-year-old male parr, increased swimming exercise and increased salinity were associated with higher thyroid activity Increase in ambient I¹²⁷ depressed thyroid activity and indicated that the activity of the gland (assessed by current methods) is partly a compensation for low l¹²⁷ availability. The high thyroid activity of the smolt may be due partly to endemic goitre. Since low temperature and 8-hour daylength inhibited the thyroid activity of potential migrants but did not prevent silvering, the role of thyroxin in guanine deposition under natural conditions is doubted. Possible radiohormone catabolic sites were located in metabolically active tissues including gut, kidney, liver and brain. These findings suggest a general rather than a tissue-specific role of thyroxin in metabolism. It is concluded that thyroxin may have no stimulatory role in smoltification but reflects instead the total metabolic demands on the tissues.