Ecdysone metabolism in Pieris brassicae eggs was revealed after injection of [3H]ecdysone into eggs. Labeled ecdysteroids were analyzed by HPLC. The metabolic fate of [3H]ecdysone i s similar to that observed during postembryonic development, i.e., the larval instars and pupal instar. The ecdysone molecule is affected by hydroxylations at C-20 and at C-26; 26-hydroxyecdysteroids can be further oxidized into ecdysteroid acids. In an effort to identify egg ecdysteroids, we examined the metabolism of [3Hlcholesterol injected into female pharate adults to achieve long-term labeling. In eggs, [3Hlcholesterol was converted to 20-hyd roxyecd yson oic acid , ecd ysonoi c acid , 20-hyd roxyecdysone, and ecdysone. The rates of [3Hlcholesterol incorporation into ecdysteroids increase according to the age of the eggs (from 0.06% at laying to 0.12% at hatching), while the cholesterol level remains constant throughout embryogenesis, suggesting that the embryos of Pieris brassicae can produce ecdysteroids either from cholesterol or from apolar ecdysteroids not identified by the analytical method. Pieris eggs contain amounts of ecdysteroids which increase from 1.5 nmol/g at laying to 3.8 nmol/g at hatching.