Approximately half of the neurons in the abdominal nervous system of the Bombyx mori die after adult eclosion. Two physiological signals regulate post-eclosion neuronal death in adult moths. The one is a steroid hormone, 20-hydroxyecdysone, and the other is a neural signal originated in nervous system. The neural signal is transmitted from the fused pterothoracic ganglion to abdominal ganglion A3 via the intersegmental connectives. In order to characterize the signal of neural origin, in vitro culture system was developed. Treatments of 20-hydroxyecdysone and drugs revealed the delay or the blockage of neuronal cell death in B. mori nervous system. Aqueous extracts of pterothoracic ganglia were prepared and applied to cultured ventral nerve cords. These extracts showed a potent activity in killing neurons. The active component of the extract was heat-stable and trypsin-sensitive. Size fraction studies suggested that the active component has a molecular mass below 30,000Da.