Thyroidal control of the transition of myosin isoforms during flounder metamorphosis was examined by the administration of either only thiourea (TU), a potent inhibitor of thyroid hormone synthesis, or thyroxine (T4) together with TU into premetamorphic larvae. Immersion of premetamorphic larvae in 400 microM of TU inhibited the appearance of the adult-type DTNB (5,5'-dithio-bis-nitrobenzoic acid) light chain, LC2, whereas administration of T4 with TU induced a precocious appearance of LC2 and decreased the relative amount of the larval-type DTNB light chain, LC2*. TU was administered into juveniles just after completion of metamorphosis. The treatment did not affect the composition of the myosin light chains. These results suggest that thyroid hormone irreversibly turns on the switch for transition of the DTNB light chains from larval to adult type during metamorphosis of the flounder.