Sex pheromone production in females of the corn earworm,Helicoverpa zea(Boddie), starts with the onset of scotophase and when the titer of the pheromone reaches ≍30 ng (usually within 30 min), females start calling. Brief exposure of scotophase females to light increased calling significantly, with a corresponding decrease in pheromone titer. Females in photophase can produce pheromone if injected with the pheromone biosynthesis activating neuropeptide (PBAN), but the titer declines rapidly after 1 h. The titer continues to increase in scotophase females for 4 h. The observed lack of pheromone production at low temperatures (14°C) is probably attributable to inhibition of PBAN release and to lower pheromone biosynthetic activity.