To understand the environmental factors and endocrine mechanisms which regulate the extended period (2 to 4 years) of larval development in Ascaphus, the effect of temperature and thyroxine (T4 by immersion) on metamorphic changes of the tail, limb buds, operculum, anal fold, oral disc, and teeth were studied in premetamorphic and prometamorphic tadpoles. (1) Control tadpoles did not undergo metamorphic change at any of the temperatures tested. (2) When tadpoles were kept in thyroxine solutions at 5 degrees, there were no changes in developmental morphology, and tadpoles kept at 18 and 20 degrees showed significant metamorphic changes in 5 to 6 days, but there was a high mortality at the higher T4 concentrations. The optimum temperature is about 10 degrees, and significant changes were completed in 3 weeks. (3) The premetamorphic tadpoles were less sensitive to T4 than prometamorphic tadpoles. The most responsive tissues are teeth and operculum, and the tail tissue shows the lowest sensitivity. (4) Duration of exposure to hormone and concentration also affect the growth or regression of larval tissue.