To test the effect of thyrotropin-releasing hormone (TRH) on serum arginine vasopressin (AVP) in euthyroid and hypothyroid individuals, 500 µg of TRH was administered to four euthyroid subjects and three patients with primary hypothyroidism. Serum AVP was significantly depressed below basal levels from 30 to 60 min in the euthyroid and hypothyroid subjects after administering the releasing agent. The basal serum AVP in the hypothyroid subjects (3.1 ± 1.1 µU/ml) was not significantly greater than the basal serum AVP in the euthyroid subjects (2.8 ± 1.2µU/ml). The maximal incremental depression in serum AVP after TRH in the hypothyroid subjects (1.4 ± 0.7 µU/ml) was similar to the depression observed in the euthyroid subjects (1.8 ± 0.9 µU/ml). These data suggest that TRH may have a physiologic role in modulation of AVP releas