Cortical neurons are densely innervated by noradrenergic fibers and by intrinsic cortical interneurons containing vasoactive intestinal polypeptide (VIP). Biochemically, VIP and norepinephrine (NE) synergistically interact to stimulate the synthesis of cyclic AMP in cortical slices. Therefore, we sought physiological indices of this peptide-monoamine interaction by applying VIP and NE to single cortical neurons of the rat while recording their spontaneous discharge. VIP applied alone inhibited discharge of 24% and accelerated discharge in 20% of cortical neurons. NE alone had a predominantly depressant effect on the same neurons. However, when VIP was retested during the continuous application of subthreshold currents of NE, VIP exerted predominantly depressant effects. These synergistic inhibitions resulted even in cells previously showing excitations to VIP alone. If VIP alone was depressant, subthreshold NE further enhanced the VIP depression. Subthreshold amounts of phenylephrine, an alpha-adrenoceptor agonist, also produced comparable interactions, suggesting involvement of an alpha receptor, as in the biochemical studies. These results support a peptide-monoamine interaction in cortex that could have important ramifications for neuronal integration.