Ovalbumin was used to trigger passive systemic anaphylactic shock in guinea-pigs treated with serum provided by actively sensitized animals. Shock was characterized by bronchoconstriction and hypotension, accompanied by leukopenia and moderate thrombocytopenia. Neither aspirin, a cyclooxygenase inhibitor, FPL 55712, a peptido-leukotriene antagonist, nor their combination interfered with shock, under conditions where the selective histamine antagonist mepyramine, up to 20 micrograms/kg, suppressed bronchoconstriction. When the animals were treated with the beta-adrenergic antagonist propranolol, mepyramine lost its activity, even if combined with aspirin and FPL 55712. Lungs provided by the sensitized animals secreted histamine and formed thromboxane A2 when challenged with ovalbumin, but failed to do so when the lungs were collected after systemic shock; demonstrating that in vivo desensitization involves direct effects on the lungs. Parenchyma lung strips from the sensitized animals and lung strips and trachea from non-sensitized animals placed together in an organ bath contracted when exposed to the antigen in presence of mepyramine. The contraction of the sensitized strips was not affected by FPL 55712 nor by the lipoxygenase inhibitors nordihydroguarietic acid and BW755c, but the responses of the non-sensitized tissues were suppressed, demonstrating that, apart from peptido-leukotrienes, parenchyma lung strips from passively sensitized animals generate a leukotriene and histamine-independent contracting activity. Histamine and peptido-leukotrienes do not account for the totality of passive anaphylactic shock in the guinea-pig.