In this study, Harderianectomy (Hdx) has been shown to differentially modify circulating levels of the thyroid hormones, T4 and T3, in male and female golden hamsters exposed to low photic intensities or to moderately low temperatures. Specifically, low photic intensities depress circulating levels of T4 in both control and Harderianectomized (Hdx) male and female hamsters. In addition, T3 is decreased in both control and Hdx males but not in females, as a consequence of reduced levels of illumination. Moderately low temperatures (10°C) depress T4 in both control and Hdx males but not in females, while T3 is increased in both control male and female hamsters, and in Hdx males, but not Hdx females. The data suggest that the Harderian gland of males enhances the sensitivity of the TSH-thyroid axis to photic intensity and to lower temperatures, and further, that there is a considerable sexual difference in the role the Harderian glands may play in response to these two environmental factors.