Plasma thyroxine (T4) levels were measured by radioimmunoassay in a spontaneously metamorphosing population of green frog, Rana clamitans, tadpoles. In accordance with species previously studied, a sharp rise in plasma T4 was seen at metamorphic climax, followed by a decline at final transformation. Unlike other species studied thus far, T4 levels were also moderately high in growing tadpoles (stages X-XIII). These early elevated levels may be due to daily rhythmicity in thyroid secretion coupled with the restricted hours of blood collection used in this study.