This study describes changes in the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the toad Bufo marinus during development from early embryonic stages through metamorphosis. Both the total content and concentrations of T4 and T3 were calculated to assess whether changes in these hormones might derive from changes in body mass or water content rather than from changes in hormone production or metabolism. Three clutches of eggs were collected from ponds during the day following fertilization and raised through metamorphosis. Samples of five or more individuals were collected daily for thyroid hormone measurement and body mass determination. Hormones were measured by radioimmunoassay utilizing miniature Sephadex columns after whole-body extraction. Both T4 and T3 were elevated at gastrulation (1 day after fertilization), but declined to low levels within the first week. This is the first report that thyroid hormones occur in anuran embryos prior to thyroid differentiation. This suggests that thyroid hormones are deposited in the anuran egg during oogenesis in a manner similar to that described for teleost fishes and the domestic hen. Levels of T3 and T4 began to rise at about 2 weeks after fertilization (stage 31; staged according to Limbaugh and Volpe, 1957) and peaked during early metamorphic climax (stage 43). This pattern coincides well with the notion that thyroid hormones are central regulators of metamorphosis in B. marinus as has been shown in other amphibians examined to date. Generally, both total content and concentration of the two hormones varied in parallel. Overall, whereas individual mass and water changes may affect the magnitude of hormone changes, they appear to have little input into their direction.