The effects of H2-blocking agents and the H2 receptor agonist, 4-methylhistamine, on the severity of anaphylactic reactions were studied in the guinea pig in vivo. The increase in gas volume of the lungs 90 sec after intravenous infusion of ovalbumin in animals immunized previously by intraperitoneal ovalbumin injection was used as an index of the severity of the reaction in vivo. The H2 receptor antagonists burimamide (1.0 and 3.0 mg per kg) and metiamide (3 mg per kg) significantly increased the severity of the reaction but did not significantly alter the effects of subcutaneous histamine. Neither 3 nor 30 mg of cimetidine per kg increased the severity of the reaction, and the higher dose significantly blunted the response to subcutaneous histamine. The H2 receptor agonist, 4-methylhistamine, significantly diminished the severity of the reation. These experiments demonstrate that H2 receptor stimulation may act to limit the severity of the anaphylactic reactions in vivo.