Adult female turtles (Lissemys punctata punctata) were treated with pineal indoleamine melatonin (100 mug/100 g) or the antithyroid agent, methylthiouracil (100 mug/100 g) or melatonin together with methylthiouracil (100 mug of each drug/100 g) for 12 days. Melatonin alone inhibited thyroid activity as evidenced by reductions in the gland weight, follicular epithelial cell height, thyroid peroxidase, and plasma thyroxine levels. Methylthiouracil caused hyperplasia of the gland, although it inhibited thyroid activity and reduced thyroid peroxidase and plasma thyroxine levels. Melatonin together with methylthiouracil produced changes similar to those obtained with melatonin alone. The results indicate that melatonin probably exerts inhibitory effects influences on both thyrotropin release from the pituitary and the activity of the thyroid itself in turtles.