Prothoracic glands of Periplaneta americana were tested under in vitro conditions for their ability to release 3‐dehydroecdysone (3DE) besides ecdysone. 3DE was identified by RIA after conversion to ecdysone by a ketoreductase‐containing hemolymph preparation as well as in HPLC fractions of incubation medium by an antiserum with affinity to 3DE and ecdysone. The ratio of 3DE to ecdysone secreted into the culture medium was found to vary between 2:1 and 7:1, depending on the period in the last nymphal instar. Enzyme activity of 3β‐ketoreductase showed fluctuations that coincided with the hemolymph ecdysteroid titre. Homogenates of prothoracic glands do not contain ecdysone but contain 3DE. Nevertheless the glands release 3DE as well as ecdysone under in vitro conditions. Different sites where conversion of 3DE to ecdysone takes place are discussed. Arch. Insect Biochem. Physiol. 41:107–116, 1999. © 1999 Wiley‐Liss, Inc.

Prothoracic glands of Periplaneta americana were tested under in vitro conditions for their ability to release 3-dehydroecdysone (3DE) besides ecdysone. 3DE was identified by RIA after conversion to ecdysone by a ketoreductase-containing hemolymph preparation as well as in HPLC fractions of incubation medium by an antiserum with affinity to 3DE and ecdysone. The ratio of 3DE to ecdysone secreted into the culture medium was found to vary between 2:1 and 7:1, depending on the period in the last nymphal instar. Enzyme activity of 3bketoreductase showed fluctuations that coincided with the hemolymph ecdysteroid titre. Homogenates of prothoracic glands do not contain ecdysone but contain 3DE. Nevertheless the glands release 3DE as well as ecdysone under in vitro conditions. Different sites where conversion of 3DE to ecdysone takes place are discussed. Arch. Insect Biochem. Physiol. 41:107-116, 1999.