We recently demonstrated that GH and interferon-gamma (IFN gamma) act in a similar manner to prime macrophages in vitro and in vivo for enhanced superoxide anion release. In this report we investigated the physiological role of the pituitary gland and GH in in vivo priming of resident peritoneal macrophages for the synthesis of tumor necrosis factor-alpha (TNF alpha) in vitro. Compared to normal rats, hypophysectomized animals had an 83% reduction in macrophage production of TNF alpha after in vitro stimulation with lipopolysaccharide. Sham operation had no significant effect on the ability of macrophages to secrete TNF alpha in response to lipopolysaccharide. Both native pituitary-derived porcine GH (48 micrograms/rat.9 days) and native pituitary-derived rat GH (96 micrograms/rat.9 days) more than tripled the in vitro production of TNF alpha by macrophages from hypophysectomized rats (342 and 358 vs. 112 U/mg protein for placebo-treated rats, respectively). Each of these preparations of GH also increased growth more than 6-fold in hypophysectomized rats (32 and 30 g vs. 5 g in placebo controls). Heat inactivation of native pituitary-derived porcine GH significantly reduced its in vivo ability to augment both TNF alpha synthesis by macrophages and body growth. Recombinant rat IFN gamma (2000 U/rat.9 days) more than tripled the production of TNF alpha by macrophages from hypophysectomized rats (343 vs. 112 U/mg protein). In contrast to its in vivo effects, addition of GH in vitro to macrophages from hypophysectomized rats did not prime these cells for the synthesis of TNF alpha, indicating an indirect mechanism of action for GH. To further test the biological relevancy of GH with respect to synthesis of TNF alpha, hemorrhagic necrosis of TNF alpha-sensitive murine methyl-cholanthrene-induced tumors was assessed in pituitary-intact mice. Native porcine GH (133 micrograms/mouse.7 days) significantly augmented both the necrosis to tumor ratio and the hemorrhage to tumor ratio. These findings establish the physiological relevance of the pituitary gland and GH in the priming of macrophages for TNF alpha synthesis.