Vasoactive intestinal peptide (VIP) has been suggested to play a role in lactation; indeed several studies implied that VIP induces the release of PRL in the pituitary. Quantitative RNA studies from our laboratory show an increase in the VIP messenger RNA (mRNA) content in the hypothalamus of lactating rats. The purpose of this investigation is to determine which hypothalamic neurons are increasing the expression of VIP. A sensitive in situ hybridization assay employing synthetic oligodeoxynucleotide probes corresponding to specific exons of the VIP gene was used to study VIP gene expression at the neuronal level. We were able to detect VIPencoding transcripts in various brain regions including the ventrolateral thalamus, neocortex, pyriform cortex, and hypothalamus with a particularly high concentration in the suprachiasmatic nucleus. When lactating animals were compared to nonlactating animals, a 2-fold increase was observed in VIP transcripts in the suprachiasmatic nucleus. Since the suprachiasmatic nucleus is not directly associated with the physiology of lactation, the response of the VIP gene to lactation may be, in part, indirect. Taken together, our results suggest that lactation and the expression of the VIP gene are interrelated.