The response of TSH to TRH is often abnormal in patients with euthyroid endocrine ophthalmopathy and, on the evidence of the present observations, falls into different patterns. The close relationship between the euthyroid and the hyperthyroid types of endocrine ophthalmopathy and the possibility of the two forms representing in reality successive phases of the same process are pointed out. Demonstration of LATS in the serum, slightly increased serum-triiodothyronine-levels, failure of normal response to TRH and an abnormal triiodothyronine-suppression test in euthyroid ophthalmopathy may herald the manifestation of hyperthyroidism at some later time.