AbstractThe metabolism of [3H]‐ecdysone has been investigated at times of low and high endogenous ecdysteroid tit re, in early and late fifth‐instarSchistocerca gregarialarvae, respectively. Ecdysone‐3‐acetate, 20‐hydroxyecdysone, and 20,26‐dihydroxyecdysone were identified as metabolites in both the free form and as polar conjugates. Comparison of the intact polar conjugates of the ecdysteroid acetates on two HPLC systems with the corresponding authentic compounds indicated that they were 3‐acetylecdysone‐2‐phosphate and 3‐acetyl‐20‐hydroxyecdysone‐2‐phosphate. Other major polar metabolites were identified as ecdysonoic acid and 20‐hydroxyecdysonoic acid. Ecdysone metabolism in fifth‐instar S.gregariais apparently an age‐dependent process. Early in the instar, excretion of both free and conjugated ecdysteroids, as well as ecdysteroid 26‐acids, occurs. At this stage the level of ecdysteroid acetates in the conjugated (phosphate) form is high, in contrast to the free ecdysteroids, where ecdysone predominates. When the endogenous hormone titre is high, the formation of ecdysteroid acetates is less, the major excreted matabolites at that stage being conjugated 20‐hydroxyecdysone together with ecdysteroid‐26‐acids, but little free ecdysteroids. Acetylation of ecdysone occurs primarily in the gastric caecae. Ecdysone‐3‐acetate (mainly as polar conjugate) is also a major product of ingested ecd