Vasoactive intestinal peptide (VIP), a 28-aminoacid peptide, plays a multifunctional neuromodulatory role in both peripheral and central nervous systems. We have recently reported that VIP induces interferon (IFN) a/p synthesis in human colon adenocarcinoma cell line HT-29. It has been reported that VIP may counteract HIV-induced neuronal cell death; therefore, we postulated that the action of VIP may be mediated by a cascade regulation, involving the production of some cytokines such as IFN. Here we demonstrate that primary cultures of rat mesencephalic neurons and glial cells respond differently to VIP. Thus VIP enhanced 2'5' oligoadenylate (2'5'A) synthetase activity and inhibited vesicular stomatitis virus multiplication in glial cultures only. However, both cell cultures had functional adenylate cyclase coupled receptors for VIP. The increase in 2'5'A synthetase activity in glial cultures reached a maximum with 1 O-' M VIP and required cellular RNA and protein synthesis. Anti-IFN a@, but not anti-IFN y, antibodies abolished the induction of the antiviral and 2'5'A synthetase activities by VIP in rat glial-enriched cultures, suggesting that these inductions were mediated through IFN a/p synthesis. Moreover, VIP or poly (i).poly (C,,U) caused, in the glial cultures, the induction and secretion of an IFN of type a/p with a titer value of I6 and 32 units/ml respectively. in contrast, neither of these two substances was able to induce IFN synthesis in neurons, which were, however, sensitive to IFN dp produced by VIP-treated glial cells. IFN produced by VIP in glial cells may therefore play an important role in defending the brain against viruses.