Vasxular permeability to Evans blue dye and 131-I-labeled human serum albumin was studied in normal mice and in mice treated with alkaline saline extracts (SE) from Bordetella pertussis cells. Skin sites inoculated intracutaneously with small doses of histamine, serotonin, or a combination of these 2 substances were more permeable in SE-treated mice than in normal animals. Intravenously administered catecholamines were able to reduce in varying degrees the vascular permeability induced by serotonin or by histamine in normal mice; in SE-treated mice the catecholamines were less effective. The relative effectiveness of intravenously administered catecholamines to reduce vascular permeability in normal or SE-treated mice was ranked as follows: isoproterenol greater than epinephrine greater than norepinephrine. When catecholamines were given concomitantly with histamine and serotonin in the skin test site, the permeability in both normal and SE-treated mice was again reduced or blocked, but isoproterenol was only weakly effective in this instance. Their relative effectiveness was epinephrine greater than norepinephrine greater than isoproterenol. The possible explanations for these results are discussed.