Activity-dependent neurotrophic factor is a potent, neuroprotective molecule released from astroglia following stimulation by vasoactive intestinal peptide and, at least in part, accounts for the neuroprotective actions of vasoactive intestinal peptide. As well as enhancing neuronal survival, vasoactive intestinal peptide is known to regulate embryonic growth during the early postimplantation period of development. The current study was designed to assess activity-dependent neurotrophic factor's role in the growth-regulatory properties of vasoactive intestinal peptide. Treatment of whole cultured day-9 mouse embryos with activity-dependent neurotrophic factor (10(-13) M) resulted in a growth of 3.1 somites, compared with 1.6 somites in control embryos after a 4 h incubation period. Significant increases were also seen in cross-sectional area, protein and DNA content and bromodeoxyuridine incorporation. Activity-dependent neurotrophic factor-treated embryos were morphologically indistinguishable from control embryos of the same size. Anti-activity-dependent neurotrophic factor ascites significantly inhibited growth. In addition, co-treatment of embryos with anti-activity-dependent neurotrophic factor ascites inhibited vasoactive intestinal peptide-stimulated growth. Although anti-vasoactive intestinal peptide treatment inhibited growth, it did not inhibit activity-dependent neurotrophic factor-induced growth. These data indicate that an activity-dependent neurotrophic factor-like substance is an endogenous and potent growth-promoting factor in the early postimplantation embryo and that vasoactive intestinal peptide-regulated growth of embryos occurs, at least in part, through the action of activity-dependent neurotrophic factor.