Several attempts were made to elucidate the possible role of histamine, serotonin, leukotrienes C₄ (LTC₄) and D₄ (LTD₄), and prostaglandin E₁ (PGE₁) as vascular permeability increasing factors involved in 48-hour homologous passive cutaneous anaphylaxis (PCA) in the mouse ear. Increased vascular permeability in the mouse ear caused by the mediator injection or PCA was assessed quantitatively by measuring the amount of extravasated dye. In skin reactions, all of the mediators used in the present study significantly increased vascular permeability. The most potent mediator was serotonin, which increased the vascular permeability from a concentration of 10^––8 g/ml, and the activity was about 100 times higher than that of histamine on a weight basis. Vascular permeability increasing activity of LTC₄ was about 10 times higher than that of histamine, and LTD₄ and PGE₁ were also more potent than histamine. Increases of vascular permeability caused by histamine, serotonin, LTC₄ and LTD₄ were significantly potentiated by injecting 10^––6 g/ml of PGE₁ simultaneously. Histamine-, serotonin- and LTC₄-induced skin reactions in the mouse ear were suppressed significantly by the administrations of chlorpheniramine, methysergide and FPL 55712, respectively. In contrast, though chlorpheniramine and methysergide suppressed also mouse ear PCA (about 50 and 40%, respectively), neither FPL 55712, indomethacin nor BW 755C suppressed it. These results strongly suggest that the most important mediator involved in mouse ear PCA is histamine and that serotonin also plays an important role in the increase of vascular permeability caused by PCA. Despite their potent vascular permeability increasing activity LTC₄, LTD₄ and PGE) do not seem to play an important role in mouse ear PCA.