Thirty male weanling Sprague-Dawley rats were fed either a copper deficient diet (0.85 mg Cu/kg) or a copper-adequate diet (8 mg Cu/kg). After 7 weeks, the rats were fasted for 12 hours, and injected intravenously with thyrotropin-releasing hormone (TRH, 30 ng/100 g body weight). Six rats from each treatment were killed at 0, 0.5, 1, 2 and 4 hours after TRH injection. Sera obtained at 0, 0.5 and 1 hour, and at 0.2 and 4 hours were used for the radioimmunoassay of thyroid-stimulating hormone (TSH) and thyroxine (T4), respectively. Reduction in liver copper content confirmed that the rats fed the test diet were copper-deficient. Serum TSH levels appeared to have peaked at 30 minutes but declined to a level higher than basal at 1 hour. No difference in TSH response was observed between the 2 treatments. Serum T4 response to TRH was reduced in the copper-deficient as compared to the adequate rats at all time intervals. After 2 hours a slight elevation was observed in the controls, but marked elevations in T4 were observed in both treatments at 4 hours. This reduction in T4 levels could be due to an impaired T4 synthesis or release in copper-deficient rats.