We recently reported that the cytokine interleukin-6 (IL-6) is a potent stimulator of anterior pituitary hormone release in vitro. Since IL-6 is not normally detectable in the blood, we hypothesized that IL-6 may be produced by the anterior pituitary in situ and thereby affect hormone secretion through paracrine or autocrine mechanisms. The present study demonstrates that cultured anterior pituitary cells spontaneously secrete large quantities of IL-6 in vitro. IL-6 was detectable in the incubation medium within 2 h, and by 8 h of culture had attained concentrations of 2000–4000 U/ml-4 × 10⁵ cells. IL-6 production was stimulated by phorbol myristate acetate (10–100 nM) approximately 2-fold and by lipopolysaccharide (0.001–10.0 μg/ml) 4-fold during 4-h incubations. In contrast, the cytokine recombinant human IL-la had no effect on IL-6 release by cultured pituitary cells. Freshly dissected hemipituitary tissue also secreted more than 3000 U/ml IL-6 during a 4-h incubation. This secretion was enhanced 3-fold by 10 μg/ml lipopolysaccharide. Our results suggest that the anterior pituitary may produce IL-6 in situ, where it may function as an intrapituitary releasing factor. (Endocrinology126: 582–586, 1990)