In the present study, we investigated the production of growth hormone (GH)-related RNA and protein in vivo by rat leukocytes after intraperitoneal treatment with different inducing agents including bacterial lipopolysaccharide (LPS) and Freund's complete adjuvant (FCA). The data showed that in rats after exposure to LPS or FCA leukocytes obtained from the spleen, thymus, and peritoneum all showed a dose-dependent increase in GH-related RNA content. The peak production of GH-related RNA was observed 48 h after treatment in the spleen and thymus and 96 h after treatment in the peritoneum. We also evaluated the ability of LPS-sensitive (C3HeB/FeJ) and resistant (C3H/HeJ) inbred mice treated with LPS to produce GH-related RNA. The LPS-sensitive mice presented with a typical pathophysiologic response pattern and higher levels of GH-related RNA in the spleen and thymus than the LPS-resistant mice. An increase in the production of immunoreactive GH (irGH) was also observed by direct immunofluorescence with specific antibodies to rat GH. We validated that the GH-related RNA produced in vivo by leukocytes was similar in structure to pituitary GH RNA using reverse transcription and the polymerase chain reaction (PCR). A sample of the PCR reaction, analyzed by gel electrophoresis, showed a single major DNA band corresponding in length (600 base pairs) to the distance between the 5′-ends of the two GH-specific primers that was specifically detected with a GH-specific probe after Southern transfer. In other studies with normal nontreated animals, the GH RNA levels are higher in the evening hours and early on in the first month of life. Taken together, our data are the first demonstration that GH RNA and immunoreactive protein can be detected in leukocytes in vivo both in normal and stimulated animals and support the idea that GH may be active in an immune response. © 1991.