The daily food intake of a large dose of retinol (as palmitate) or retinoic acid (as all-trans retinoic acid) modifies numerous parameters of the thyroidal status in the rat. There were decreases of the serum total thyroxine (TT₄) and triiodothyronine (TT₃), and of the biological half-life of T₄, while there were increases of the dialyzable fractions of T₄ and T₃ and of the apparent distribution space of T₃. The possible causes of these changes are discussed.