Bombyxin is a 5 kDa insulin-related peptide produced in four pairs of medial neurosecretory cells in the brain of the silkmoth Bombyx mori. We demonstrate here the presence of bombyxin mRNA in tissues other than brain: ganglia, epidermis, testis, ovary, fat body, silk gland, Malpighian tubule, midgut, and hindgut of the Bombyx fifth instar larvae. Bombyxin mRNA was detected by Oligotex reverse transcription-polymerase chain reaction (RT-PCR), a rapid and simple procedure of reverse transcription-PCR, and in situ hybridization. The Oligotex RT-PCR method effectively eliminated the contaminating DNA in RNA samples and amplified bombyxin mRNA efficiently. In situ hybridization of the Bombyx ovary clearly demonstrated the localization of the bombyxin mRNA in the ovariole. The present study is the first demonstration of expression of brain neurosecretory peptide in tissues other than the central nervous system in insects at RNA level.