We measured the circulating T4 and T3 levels in the plethodontid salamander Eurycea bislineata at various stages of metamorphosis using radioimmunoassay (RIA). Seven distinct metamorphic stages were defined based on specific developmental events concerning the remodeling and differentiation of skeletal elements. Special effort was made to study individual variation in the levels of plasma thyroid hormones. For this reason we did not pool serum from several specimens. The RIA was conducted in aliquots of 2 microliters (T4) and 5 microliters (T3), with minimum detectable levels of 100 ng/dl (T4) and 20 ng/dl (T3). In agreement with previous studies on other amphibians, we found metamorphosis in E. bislineata to be accompanied by a sharp increase in the circulating plasma levels of T3 and T4. No hormones were detectable in the larval and adult stages. Our technique allowed for simultaneous measurement of T3 and T4 levels in some individuals. These data indicated that, although both the onset of the production of the two thyroid hormones is simultaneous, T3 remains in the system longer than T4. However, at all metamorphic stages a large proportion of specimens did not exhibit any measurable levels of T3 and/or T4. These results underscore the need to reassess the mode of operation and production of thyroid hormones in amphibian metamorphosis.