In the present study, we evaluated whether mononuclear leukocytes could synthesize and secrete growth hormone-releasing hormone (GHRH) in vitro. By using RNA slot-blot analysis, we detected maximum basal levels of specific GHRH mRNA in the cytoplasm of rat leukocytes after an 8 h in vitro incubation. Northern gel analysis demonstrated that the specific GHRH RNA was polyadenylated and had a molecular mass of approximately 0.8 kDa. Further studies using antibody affinity chromatography followed by size separation on high-performance liquid chromatography (HPLC) columns showed two peaks of immunoreactive (ir) material, a large molecular weight species, and a smaller molecular weight species at approximately 5 kDa. The smaller molecular weight irGHRH appeared to be de novo synthesized since it could be radiolabeled with tritiated amino acids. Both molecular species were detectable in enzyme-linked immunosorbent assay (ELISA) with specific antibodies made to the first 23 amino acids as well as specific antibody obtained commercially made to the entire molecule (1-43). Although the larger molecular weight form appeared to be the more predominant, only the lower molecular weight form could block the binding of 125I-hGHRH to pituitary cells. Most importantly, the lower molecular weight leukocyte-derived GHRH stimulated an increase in the level of GH RNA in the pituitary. We conclude that lymphocytes produce an irGHRH that is similar to hypothalamic GHRH in terms of bioactivity, antigenicity, and molecular weight. The findings demonstrate a potential regulatory loop between the immune and neuroendocrine tissues. © 1990.