Serotonin is a neurotransmitter functioning also as a hormone and growth factor. To further investigate the biological role of serotonin during embryo development, we analysed serotonin localization as well as the expression of specific serotonin 5-HT1D receptor mRNA in mouse oocytes and preimplantation embryos. The functional significance of serotonin during the preimplantation period was examined by studying the effects of serotonin on mouse embryo development. Embryo exposure to serotonin (1 microM) highly significantly reduced the mean cell number, whereas lower concentrations of serotonin (0.1 microM and 0.01 microM) had no significant effects on embryo cell numbers. In all serotonin-treated groups a significant increase in the number of embryos with apoptotic and secondary necrotic nuclei was observed. Expression of serotonin 5-HT1D receptor mRNA in mouse oocytes and preimplantation embryos was confirmed by in situ hybridization showing a clearly distinct punctate signal. Immunocytochemistry results revealed the localization of serotonin in oocytes and embryos to the blastocyst stage as diffuse punctate cytoplasmic labelling. It appears that endogenous and/or exogenous serotonin in preimplantation embryos could be involved in complex autocrine/paracrine regulations of embryo development and embryo-maternal interactions.