Annetocin is structurally related to an OT (oxytocin)/VP (vasopressin) family peptide, which has been isolated from the earthworm Eisenia foetida and has been shown to induce OT-like egg-laying behaviour. We now report the identification of an endogenous AnR (annetocin receptor). The deduced AnR precursor displays high sequence similarity with OT/VP receptors. Genomic analysis of the AnR gene revealed that the intron-inserted position is conserved between the AnR gene and the mammalian OT/VP receptor genes. These results indicate that AnR and mammalian OT/VP receptors share a common ancestor gene. Administration of annetocin to the AnR expressed in Xenopus oocytes induced a calcium-dependent signal transduction. Reverse transcriptase-PCR analysis and in situ hybridization showed that the AnR gene is expressed specifically in the nephridia located in the clitellum region, although the nephridia are distributed throughout the worm body. This result suggests that annetocin induces egg-laying behaviour through its action on the nephridia. This is the first description concerning the functional correlation between an invertebrate OT/VP-related peptide and egg-laying behaviour.