In order to compare ecdysone metabolism between diapause eggs and non-diapause eggs of the silkworm, Bombyx mori, ³H-ecdysone and its derivatives (³H-3-epiecdysone and ³H-ecdysone 22-phosphate) were injected into the eggs at various stages during early embryogenesis, and the resultant labelled metabolites were analyzed by high-performance liquid chromatography. From the quantitative and qualitative changes in the labelled metabolites between diapause eggs and non-diapause eggs, it was demonstrated that epimerization of ecdysone occurred during early embryogenesis irrespective of the embryonic stage in both diapause eggs and non-diapause eggs, and that phosphorylation of ecdysone was a major metabolic step in diapause eggs, whereas dephosphorylation of ecdysone 22-phosphate and its subsequent hydroxylation at the C-20 and C-26 positions were characteristic in non-diapause eggs.