Summary: Urethane (and chloralose) in vitro inhibit anaphylactic contraction of sensitized plain musculature (uterine strips) of guinea pigs. This phenomenon may explain the mode of action of urethane in the prevention of fatal anaphylactic shock in guinea pigs. Although the presence of urethane inhibits this anaphylactic contraction of plain musculature, it does not prevent the union of antigen and antibody. These phenomena confirm the correctness of Dale's “two-stage” conception of anaphylaxis.