The aim of the present study was to clarify the role of feedback control on the (brain)-pituitary-gonadal axis in regulating FSH (gonadotropic hormone, GTH I) and LH (GTH II) in natural maturation in salmonids. In two experiments, 2-yr-old previously mature Atlantic salmon male parr were castrated or sham operated in the spring following their first reproductive season. In one of the experiments, castrated fish were also implanted with silicone elastomer capsules containing testosterone (T) or

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... 1ketoandrostenedione (11 KA). The fish were sampled in July, September, and November (spawning period). Pituitary and plasma LH and FSH levels were measured using RIA and were lower in castrated than in sham-operated fish, indicating positive feedback on both FSH and LH. T, and to a lesser extent 11KA, increased pituitary LH content in castrated fish. The 11KA increased plasma and pituitary FSH levels, whereas T suppressed FSH in July and stimulated it in November. Plasma FSH levels peak earlier than LH, and it is suggested that if one or more feedback effects are involved in controlling the "all-or-nothing response," i.e., whether a fish will mature or not, a feedback effect on FSH is the most likely candidate.