A significant increase was observed in thyroid radioiodine uptake in goldfish with an autotransplanted pituitary compared to the intact controls. This suggests that the pituitary secretes TSH at a higher rate following transplantation, and supports the idea that TSH secretion is normally under the control of an inhibitory factor from the hypothalamus in goldfish. Following implantation of a pellet containing 0.1–0.5 µg thyroxine (T4) into the autotransplanted pituitary, there was no suppression of radioiodine uptake. There was a significant suppression of radioiodine uptake following implantation of a pellet containing 1.0–1.5 µg T4 into the autotransplanted pituitary, but no suppression occurred when this larger dose of T4 was implanted in the optic tectum, or nucleus lateralis tuberis region of the hypothalamus, of fish with an autotransplanted pituitary. This indicates that T4 has a direct negative feedback effect on the pituitary in goldfish.