SYNOPSIS. This review examines the way in which the activity of the hypothalamo-pituitary-thyroid axis in teleost fishes is modified by pituitary, steroid and amine hormones. These factors may act at the level of the hypothalamus, the pituitary gland (thyrotrops), the thyroid gland, or the peripheral tissues, and affect thyroid hormone synthesis, release, metabolism, or degradation. With few exceptions, the studies are limited to only a few species, the results are often fragmentary and contradictory, are based on experiments in which pharmacological, rather than physiological, hormone ranges have been used, and there is too little information to establish a consistent pattern of response to endocrines other than those which are components of the hypothalamo-pituitary-thyroid axis itself. Most studies evaluating the effects of pituitary hormones on thyroid economy in fish have, of necessity, relied on mammalian preparations. Some of these (e.g., prolactin and the gonadotropins) elicit very different responses from those of the equivalent semipurified or purified piscine hormone, although mammalian and piscine TSH and GH respectively appear to elicit similar responses in the teleostean thyroid system. The elevation of plasma T4 levels in response to a challenge of exogenous TSH has been used in several studies as a way of evaluating the sensitivity of the thyroid to TSH; the response is modified by photoperiod, season, stage of development, estrogen and corticosteroids.