Intravenous injection of synthetic thyrotropin-releasing hormone (TRH) resulted in a significant and dose-related increase in plasma growth hormone (GH) in the urethane anesthetized rat. The minimum effective dose of TRH was 40 ng per 100 g body wt when administered intravenously. The maximum responses of plasma GH to TRH were observed at 5 min following TRH, whereas those of plasma TSH were obtained at 10 min. Pretreatment with either T3 (50 ¼g/100 g bw ip) or T4 (50 ¼g/100 g body wt ip) significantly suppressed both plasma GH and TSH responses to TRH. The GH release induced by TRH was significantly inhibited by somatostatin (5 ¼g/100 g body wt iv) which was injected 5 min and immediately before the injection of TRH. (Endocrinology96: 1114, 1975)