Gill (Na+_K+)-ATPase activity, triiodothyronine (T3) and thyroxine (T4) circulating blood plasma levels were monitored in 1983 on yearling Atlantic salmon reared in a hatchery in Brittany. The level of enzyme activity increased between February and April to reach a high plateau between mid-April and mid-May. The same fish presented an acute surge of T4 in mid-April and two small increases of T3 in February and May. The gill enzyme activity and the T4 blood plasma level follow a similar pattern, reaching high values at the same moment. The T4 peak occurs at the new moon and does not seem to be correlated with an increase of temperature. Possible relationships between blood plasma T3, T4 and gill (Na+_K+)-ATPase activity are discussed.