Changes in circulating L-thyroxine and L-triiodothyronine of the masu salmon, Oncorhynchus masou accompaning the smoltification, measured by radioimmunoassay.

Serum levels of L-thyroxine (T4) and L-triiodothyronine (T3) of the masu salmon, Oncorhynchus masou, were measured by radioimmunoassay during the months from March to September. Serum levels of T4 and T3 varied from 0.06 to 1.44 microgram and from 0.06 to 0.35 microgram per 100 ml serum respectively, although T4 content could not be detected in some specimens. T4 levels showed a seasonal fluctuation in which a higher value was observed in April when parr-smolt transformation was proceeding. T4 level in spring, particularly in April and June, was significantly higher in smolt than in parr. T3 content showed a small variation during the observed months and did not increase at smoltification, although the content tended to decrease to a minimum level in September when T4 level showed also the lowest level.