Levels of 1 I -0xotestosterone (1 7 P-hydroxyandrost-4-ene-3, I 1 -dione) and testosterone in the blood serum of individually marked adult male Atlantic salmon held in captivity, were measured by radioimmunoassay at approximately monthly intervals for periods of up to 18 months. In addition to peak concentrations of both hormones shown by all the maturing fish at the time to full sexual maturation during October and November, a majority of maturing fish also showed a significant elevation of 1 1 -o,xotestosterone during the early months of the year. The possible involvement of this early elevation of 1 1-oxotestosterone in controlling the mitotic multiplication of spermatogonia is discussed. Weight and length increases expressed as specific values G w and GL and weight to length relationships for the maturing males for each sampling period are presented and compared with those of nonmaturing fish.

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