The secretory activity of the harderian gland (hg) in the frog Rana esculenta is influenced by pituitary and thyroid hormones. Administration of thyroid stimulating hormone (TSH), T3 and T4 to hypophysectomized male and female frogs, affects the release of secretory granules in the lumina of glandular acini. In particular, treatment with T3 radically modified morphological parameters (height and area of acinar lumina of the glandular cells); T4 or TSH had less effect. Administration of thiouracil counteracted TSH effect, restoring the secretory content of the glandular cells. Ultrastructural observations confirmed these results. Our data suggest the existence of a thyroid-hg interaction and that thyroid hormones may be involved in the mechanism of release of mature secretory granules in the hg of the frog, Rana esculenta.