Rhythmicity of thyroid follicle structure, cell division, and T4 secretion in vitro was studied in anuran tadpoles on a 12L:12D cycle with light onset at 0800 hr. Stage XIII Rana pipiens larvae had a significant circadian rhythm of follicle lumen diameter (acrophase 1052 hr). Follicle cell height was higher at 0900 hr than at other times but the rhythm was not significant. At Stage XX, both follicle cell height (acrophase 2318 hr) and cell division (acrophase 1929 hr) were rhythmic while lumen diameter, though highest at 0900 hr, showed no significant daily changes. Thyroids from Rana catesbeiana larvae at Stages XVII to XVIII had a significant circadian rhythm of T4 secretion (acrophase 2034 hr) in vitro as measured by radioimmunoassay of media collected every 6 hr for 24 hr. The findings indicate that the daily sequence of thyroid structure and function in preclimax Rana tadpoles includes larger follicle lumina early in the light and maximum cell division, T4 secretion, and follicle cell height within a 4-hr interval beginning around the onset of dark.