Lymphocyte migration from the blood to sites of tissue injury is mediated, in part, through the interaction of these cells with endothelial cells lining the vessel walls. The ability of endothelial cells to produce nitric oxide may be important in this process. We found that the addition of the nonspecific lymphocyte activators lipopolysaccharide (LPS) or concanavalin A (Con A) to rat hepatic endothelial cell cultures from control or endotoxemic rats markedly enhanced the ability of these cells to produce nitric oxide. In contrast, wheat germ agglutinin (WGA) and phytohemagglutinin (PHA) had no effect on nitric oxide release. Coculture of endothelial cells with lymphocyte-rich preparations of rat thymocytes or splenocytes stimulated endothelial cell nitric oxide production. This response was enhanced by LPS or Con A and to a lesser extent by WGA or PHA. In contrast to endothelial cells, thymocytes and splenocytes did not produce nitric oxide either in the presence or absence of lymphocyte mitogens. Increased production of nitric oxide by endothelial cells in response to lymphocytes and lymphocyte mitogens was due, at least in part, to increased expression of protein for an inducible form of nitric oxide synthase, as measured by Western blotting. Stimulation of endothelial cell nitric oxide production by thymocytes and splenocytes was inhibitable by the specific nitric oxide synthase inhibitor NG-monomethyl-L-arginine and dependent on cell-cell contact. Thus, nitric oxide production by endothelial cells was reduced when the lymphocytes were physically separated from the endothelial cells using cell culture inserts. We hypothesize that nitric oxide released by endothelial cells increases vascular permeability, thereby allowing the extravasation of lymphocytes into the surrounding tissue, a process that may be important in inflammation, tissue injury, and/or wound healing.