The timing and magnitude of the pupal commitment peak in the hemolymph ecdysteroid titer of fifth instar Manduca sexta larvae are controlled by the combined effects of prothoracicotropic hormone (PTTH), a prothoracic gland-stimulating factor present in the hemolymph, and the biosynthetic competence of the prothoracic glands themselves. The present data indicate those individual effects are coordinated by juvenile hormone (JH): (1) Treatment of larvae with the JH analog (7S)-hydroprene prevents the normal precommitment drop in the titer of the stimulatory factor; (2) treatment of larvae with (7S)-hydroprene suppresses in a dose- and time-dependent manner the biosynthetic competence of the prothoracic glands; and (3) (7S)-hydroprene acts directly on the brain to inhibit the release of PTTH in vitro. Thus, during Manduca development, a drop in the JH titer early in the fifth instar results in a rapid drop in the titer of the stimulatory factor, the gradual acquisition by prothoracic glands of biosynthetic competence, and lastly, the gated release of PTTH into the hemolymph. The resulting increase in ecdysone synthesis by the prothoracic glands gives rise to the small peak in the ecdysteroid titer that drives pupal commitment. © 1987.