The central nervous system of Agrius convolvuli is composed of brain, followed by subesophageal ganglion, three thoracic ganglia, and eight abdominal ganglia in late larval stage. After metamorphic transition from larva to pupa, thoracic ($T_1;and;T_2$) and abdominal ganglia ($A_1;and;A_2$) are moved toward $T_3$ and fused together to construct composite ganglion, pterothoracic ganglion. The formation of composite ganglion is completed about 90% at 4 day and 100% at 7 day after pupation. Neuronal cell death was occurred significantly around 3 or 4 day after pupation and just after adult ecdysis. Although 170 neurons were detected 3 day before adult ecdysis, 24 cells were counted 5 day after adult ecdysis. Data of scanning and tandem electron microscope showed the symptom of cell death. In order to identify the mechanism of cell death in A. convolvuli, 1,200 ventral nerve cords were homogenized. Extracts were boiled for 3 minutes at $100^{circ}C$ and collected below 30,000 dalton of molecular mass. Each fraction from reverse phase column chromatography by HPLC system was tested in ventral nerve cord culture system, and fractions having killing activity in culture were isolated. By the addition of 20-hydroxyecdysone, actinomycin D, or cycloheximide into the culture medium, cell death was delayed significantly when compared to control group.