A re-evaluation of the role of prostaglandins (PGs) in releasing egg-laying behaviour in the silkmoth, Bombyx, was performed. The results revealed that PGs do not play a crucial role as oviposition behaviour stimulants. Injection of prostaglandin E₂ (PGE₂) had no effect on egg-laying behaviour of virgin females. The oviposition behaviour of females which were injected with PG biosynthesis inhibitors such as indomethacin, quinacrine and NDGA was not interrupted or suppressed. Sterilized males of Bombyx can be induced by heat treatment (32°C) for 72 hr during the wandering stage. The rate of oviposition of female moths mated with sterilized males was suppressed at a great deal, compared to the control group, in the ensuing mating period of 24 hr. PGE₂ injection into the females mated with the sterilized males had no effect on the restoration of oviposition behaviour. There were no significant difference in the PGE₂ content in the testes of normal and sterilized males. After mating, the amount of PGE₂ increased with age in the bursa copulatrix of females which had mated with either normal or sterilized males. These results indicate that PGs do not seem to be directly involved in the egg-laying behaviour of Bombyx.