What is the nature of the cellular actions of VIP (vasoactive intestinal polypeptide) in the neocortex? Is VIP co-localized with another neurotransmitter? Do VIP neurons interact with other cortical neurotransmitter systems? How does the intracortical VIP-containing neuronal system relate to the basic elements of cortical organization? Recent immunohistochemical and pharmacological investigations have addressed these questions and have shed some light on the possible function of VIP-containing neurons in the neocortex