Thyroxine (T4) concentrations in samples of fresh sera taken from fed and unfed larval (ammocoetes) and metamorphosing anadromous sea lamprey (Petromyzon marinus) were measured by means of radioimmunoassay. T4 levels in unfed ammocoetes after a 1-month period were not significantly different from those of the fed ammocoetes. Nutritional state of ammocoetes therefore does not seem to affect their serum T4 levels. Serum T4 concentrations decreased abruptly at the onset of metamorphosis and declined to levels observed in spawing adults (Leloup and Hardy, '76; Packard et al., '76) by the middle (stage 3) of metamorphosis. No significant change in serum T4 levels occurred between stage 3 to the end of metamorphosis. The possible significance of decline in serum T4 levels to the metamorphic event in lamprey is discussed.