Interleukins are proteins involved in the immune system and have been related to the endocrine regulation of the hypothalamic-pituitary-adrenal axis as well as to the secretion of ACTH, prolactin (PRL), GH and, possibly, LH. Like interleukin-6 (IL-6), vasoactive intestinal peptide (VIP) is synthesized in the pituitary gland and stimulates prolactin secretion. The aim of the present study was to address whether Interleukin 6 is involved in the regulation of VIP, as well as other factors involved in the regulation of prolactin such as dopamine, TRH and estradiol. Accordingly, we performed an in vitro study on monolayer cultures of rat pituitary cells, neutralizing the possible paracrine effect of IL-6 by immunosuppressing the protein by treatment with polyclonal antibody against IL-6 over 1, 3, 6 or 24 hours and then determining the degree of proliferation of VIP cells using double immunocytochemical labelling for VIP or PRL and proliferating cell nuclear antigen (PCNA). As a control, the effects of immunosuppression on the proliferation of PRL-positive cells were analyzed. Immunosuppression of IL-6 induced modifications in the cellular and nuclear size of VIP-positive cells, indicating an inhibitory process. Moreover, immunosuppression induced a significant decrease in the proliferation rate of PRL-positive or VIP-positive cells for all time-points analyzed. Similar effects on the proliferation rate of PRL-positive cells were found. The results of the present study demonstrate that IL-6 is involved in the regulation of the activity and proliferation of pituitary VIP-producing cells and suggest that, without ruling out a direct effect of IL-6 on prolactin cells, IL-6 could regulate prolactin by acting on pituitary VIP.