The effects of macrophages activated by bacterial lipopolysaccharides (LPS) on the mechanical activity of lymph vessels with or without the endothelium were investigated using conventional bioassay preparations. Rat peritoneal macrophages emigrated by an injection of thioglycollate were isolated and cultured for 12 h in RPMI 1640 medium containing 10μg/ml LPS. More than 97% of the cultured cells were stained with monoclonal antibody ED1 and demonstrated phagocytosis of acetylated low-density lipoprotein. The supernatant of the macrophages (Me) suppressed significantly the basal tone of the lymphatic bioassay rings precontracted by 10 8M U46619. The Me-induced vasodilation of the lymph vessels was significantly reduced by 12 h preincubation of the macrophages with 5×10-5M Nw-nitro-L-arginine methyl ester (L-NAME), 10-s M indomethacin, 10 6M dexamethasone, or 10-5M cycloheximide. Simultaneous preincubation of L-NAME and indomethacin caused a synergistic reduction of the Me-induced vasodilation of the lymphatic bioassay rings. The superfusion of Krebs-bicarbonate solution containing 5×10-5 M L-NAME, 5×10-5M aspirin, or the culture medium with no macrophages caused no significant effect on the M-induced vasodilation. These findings suggest that macrophages activated by bacterial LPS produce a marked relaxation of lymphatic smooth muscles through the co-release of nitric oxide and vasodilative prostaglandins, which may result in the facilitation of edema formation in wound tissues.