The pulmonary mechanical responses observed after antigen challenge in 2 groups of sensitized, mepyramine-treated, mechanically ventilated guinea pigs were compared: one group was fed a diet rich in fish oil and the other a control diet enriched with beef tallow. The lung tissue of animals fed a fish-oil-enriched diet for 9 to 10 wk incorporated eicosapentaenoic acid (EPA) and docosahexaenoic acid, which constituted 8 to 9% of the total fatty acid content, whereas these alternative fatty acids constituted less than 1% of total fatty acid content of the lung tissue of animals receiving a diet supplemented with beef tallow. With mepyramine pretreatment, animals receiving a fish oil diet exhibited a significantly greater decrease in dynamic compliance from 1.5 through 4.5 min after antigen challenge than did animals receiving a beef fat diet, whereas the decrements in pulmonary conductance were comparable. The combination of indomethacin and mepyramine markedly augmented the antigen-induced decrease in pulmonary mechanics in animals receiving a beef fat diet but not in those receiving a fish oil diet, such that the overall responses of the 2 groups were similar. These findings indicate that the fish oil diet and the indomethacin pretreatment of animals receiving the beef fat diet each facilitates the nonhistamine-mediated bronchoconstrictor response in pulmonary anaphylaxis.