Abstract 1. 1. Classical ligation techniques were used to determine the critical periods for the head and the prothoracic glands in the larval-pupal molt of the European corn borer. 2. 2. Most larvae reared under LD 16:8 and 30°C released sufficient PTTH between lights on + 13 hr and lights on — 3 hr to stimulate the prothoracic glands to produce ecdysone. 3. 3. Ecdysone was first released just before lights on, with almost all larvae having produced sufficient ecdysone to elicit a molt by lights on + 6 hr. 4. 4. Critical periods for the brain and prothoracic glands of Ostrinia are compared with those of other lepidopterans.