Purpose: Experiments were designed to determine whether or not leukocytes activated by acute pulmonary rejection cause contractions of isolated pulmonary arteries. Methods and Results: Separate suspensions of (a) polymorphonuclear cells (>95%) and (b) mononuclear cells (85% lymphocytes/10% monocytes/5% polymorphonuclear cells), respectively, were obtained from the arterial blood of four groups of adult male mongrel dogs: unoperated dogs (controls), dogs with single-lung autotransplants, dogs with rejecting single-lung allotransplants, and unoperated dogs treated with the same immunosuppressants as allotransplanted dogs. These suspensions were added to rings of control intralobar pulmonary arteries suspended in organ chambers for measurement of isometric force. The endothelium was removed mechanically from selected rings. No significant change in basal tension of pulmonary arterial rings occurred by adding suspensions of polymorphonuclear cells from any of the four groups of dogs. Significant cell-number-dependent increases in tension occurred with suspensions of mononuclear cells from unoperated dogs, autotransplanted dogs, and unoperated, medicated dogs. These increases in tension were less in rings with compared to those without endothelium. Addition of a synthetic analogue of L-arginine abolished this difference. Suspensions of mononuclear cells from rejecting allotransplanted dogs caused significantly greater contractions in rings with endothelium than those observed with suspended cells from either unoperated, autotransplanted dogs or unoperated, medicated dogs. Addition of superoxide dismutase plus catalase or an antagonist of endothelin-A receptors (BQ-123) reduced contractions in rings with endothelium but not in those without endothelium to suspensions of mononuclear cells from rejecting allotransplanted dogs. Conclusions: The results of this study suggest that mononuclear cells cause contraction of pulmonary arteries, which can be partially inhibited by endothelium-derived nitric oxide. However, if the mononuclear cells are activated by acute pulmonary rejection, contractions are no longer inhibited by the endothelium. Under conditions of rejection, contractions are mediated in part by oxygen radicals and endothelin(s).