AbstractFollowing i.p. injection of ovalbumin (OVA) plusBordetella pertussisvaccine into Hooded Lister rats, the time‐course of sensitization of peritoneal and lung mast cells (MC) did not parallel kinetic changes in the levels of circulating OVA‐specific and total IgE. OVA‐induced secretion of 5‐hydroxytryptamine from isolated peritoneal and lung MC and the presence of OVA‐specific IgE in serum were first demonstrated at day 14 post‐immunization. However, subsequent to day 14, the responsiveness of both types of MC to OVA declined, while circulating levels of OVA‐specific IgE continued to rise. Peritoneal MC, but not lung MC, showed increased responsiveness to challenge with anti‐IgE on day 7 post‐immunization, whereas circulating levels of total IgE were not elevated until day 14, thus demonstrating that nonantigen‐specific IgE was acquired by peritoneal MC before it entered the circulation. Lung MC generally showed decreased reactivity to both OVA and anti‐IgE, compared with peritoneal MC; no significant correlations were demonstrated between the responses of MC from t