AbstractExtracts of brains, subesophageal ganglia, Malpighian tubules, mandibular glands, proctodaea, salivary glands, testes, and fat body from mature fifth‐stage European corn borer larvae contain a 3‐oxoecdysteroid 3β‐reductase that converts 3‐dehydroecdysone to an RIA‐detectable ecdysteroid that has the same retention time (on HPLC and TLC) as ecdysone. Enzyme activity is destroyed by boiling, exposure to organic solvents, and treatment with trypsin. The reductase has a molecular weight in the range of 24–37 kD and is dependent upon the presence of NADPH for activity. Under the conditions utilized here, subesophageal ganglia, proctodaea, and brain extracts exhibited the highest specific activity; mandibular glands, testes, salivary glands, and Malpighian tubules had moderate specific activity; and fat body had the least. Based on total organ activity, however, fat body and salivary glands had the greates activity; testes, Malpighian tubules, mandibular glands, and proctodaeum had moderate activity; and brain and subesophageal ganglion h