The immunopathogenesis of the anaphylactoid Mazzotti reactions has been studied by comparing physiologic and immunologic aspects of diethylcarbamazine-induced shock in Dirofilaria immitis infected dogs with antigen induced anaphylaxis in infected and uninfected controls. Filarial antigen, specific host IgG antibody, and C1 and C3 complement levels were quantitatively measured over time in relation to the levels of histamine and prostaglandin D2 in the blood and changes in mean blood pressure. D. immitis antigen injected into uninfected dogs having no detectable IgG antibody to D. immitis or Toxocara canis produced a rapid drop in blood pressure that paralleled a drop in C1 and C3 levels and an increase in prostaglandin D2. Antigen injected into infected dogs with IgG antibody produced a similar drop in blood pressure and complement and increase in prostaglandin D2 which differed from the uninfected group only in the slower clearance of antigen from the blood. Diethylcarbamazine alone produced no measurable changes in blood pressure or complement in uninfected hosts. Diethylcarbamazine, however, administered into skin test positive infected dogs, produced a temporally slower but quantitatively similar loss in blood pressure, drop in complement, and increase in prostaglandin D2 and histamine to that induced by antigen injection. Complement activation and immune complex formation are initiated by antigen release, and subsequent vasoactive mediator release leads to shock with prostaglandin D2 being quantitatively higher in blood than is histamine.