Fish are a much studied group of early vertebrates. In addition to understanding the biology of these animal species, the research helps elucidate vertebrate evolution and dynamics of aquatic ecosystems. The research is also of importance for the establishment of successful aquaculture, and novel discoveries may aid medical research. Fish differ from other vertebrates in having indeterminate skeletal growth and significant post-natal hyperplasic muscle growth. The major endocrine growth regulatory system includes growth hormone (GH) and insulin-like growth factor I (IGF-I). In addition to stimulating growth and metabolism, GH has major effects on fish appetite and behavior, in part by a direct effect on the central nervous system. Ghrelin and leptin have relatively recently been identified in fish, and are being studied for their possible role(s) in regulating appetite and energy balance. The poikilothermic fish, in contrast to homeothermic vertebrates, can have prolonged periods of anorexia as a part of their normal physiology. Regulation of growth and osmotic balance is interlinked through the pluripotent action of the GH-IGF-I system. In anadromous fish species such as salmonids, GH is a seawater-adapting hormone, increasing hypoosmoregulatory ability. GH stimulates Na+,K+-ATPase activity of osmoregulatory tissues such as gills and intestine, together with cortisol, which in addition to its glucocorticoid function, acts a mineralocorticoid in fish. In contrast to GH, prolactin acts as a freshwater-adapting, osmoregulatory hormone in fish. This research has been supported by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS).