The type II 5'-deiodinase (5'D-II) is regulated by the light-dark cycle in some tissues in which the enzyme is present. This prompted us to investigate putative influences of light-dark cycle on thyroid hormone concentrations in these tissues. The results revealed the following facts: (a) Deiodinase activity in the rat thymus exhibits a nyctohemeral profile with peak values late at night and basal values during the day. The thyroid hormone concentrations in the thymus also show a 24 h rhythm with an increase in the triiodothyronine/thyroxine (TT3/TT4) ratio at night. (b) The content of thyroid hormones in the pineal gland exhibits, like in the thymus, nyctohemeral variations with increase values in the TT3/TT4 ratio during the dark period coinciding with the maximal enzyme activity. (c) Other tissue, like the anterior pituitary, in which 5'D-II, activity does not exhibit a diurnal variation, the concentration of thyroid hormones does not show modifications. In conclusion, the nocturnal increase of 5'D-II activity produces an increase of T3 concentration and a decrease of T4 concentration in both thymus and pineal gland. Therefore, these diurnal changes in 5'D-II activity is a mean by which the cell can regulate the intracellular availability of the most active thyroid hormone T3.