After the metamorphosis of Tenebrio molitor (Coleoptera), Acetylcholinesterase (AChE) activity per adult brain was twofold the activity of the last larval instar. During metamorphosis, from the prepupa to the adult fluctuations occurred in brain AChE activity per protein amount. These fluctuations were superimposable to the molting hormone (ecdysterone) variations in hemolymph so that each rise of ecdysterone was accompanied by an AChE peak.