The preliminary inhibition of prostanoid biosynthesis by indomethacin (10 mg/kg, intravenously) strongly changes the typical anaphylactic shock reaction produced by the intracoronary injection of the challenging dose of the antigen in the dogs sensitized with normal horse serum. A rapid fall of blood pressure and cardiac output at the first stage of the anaphylactic reaction is sharply diminished, the reaction of the coronary vessels is inversed. Instead of dilatation, vasoconstriction occurred increasing with the repeated administration of the antigen. The data obtained suggest that the initial hypotension under anaphylactic shock is due to prostanoid production while the coronary spasm is the result of lipoxygenase activation.