FACTORS controlling central nervous system (CNS) growth immediately after neurulation are mostly unknown. Vasoactive intestinal peptide (VIP) receptors are widely distributed in the embryonic nervous system^1,2, and VIP has trophic and mitogenic properties^3,4 on embryonic neural tissues but inhibits growth⁵ and mitosis in certain tumours⁶. To address the potential effects of VIP on embryonic growth, we used whole postimplantation embryo cultures^7,8. After a 4-h incubation, VIP stimulated growth, increasing somite number, embryonic volume, DNA and protein content, and number of cells in S-phase. A VIP antagonist^9,10 substantially inhibited these VIP-mediated increments in growth. The VIP antagonist completely suppressed VIP-stimulated mitosis in the CNS while decreasing the same in non-neuronal tissues by 38%. In vitro autoradiography revealed GTP-sensitive and GTP-insensitive VIP receptors which were differentially regulated in VIP antagonist-treated embryos. The present study suggests that VIP acts as a growth factor on early postimplantation embryos through multiple VIP receptors that exhibit tissue-specific responses.