Changes in chemical mediators after antigen challenge and exercise challenge tests were studied in children with asthma. Chemical mediators studied after antigen challenge and exercise challenge included histamine, leukotrienes (LTB4, LTC4, and LTD4), and neutrophil chemotactic factor of anaphylaxis (NCA). The pharmacologic modification of immediate and late-phase reactions was evaluated for procaterol (beta 2-agonist), cromolyn sodium, and prednisolone. Histamine levels were noted to rise in patients who had a dual response of a mild to moderate nature, but did not change in patients who had severe asthma. During exercise challenge cromolyn sodium inhibited both immediate and late-phase reactions but also inhibited plasma generation of NCA. Prednisolone, on the other hand, did not affect immediate reactions, but blocked late-phase reactions to exercise and also decreased NCA generation. Procaterol inhibited the generation of LTD4 and also inhibited NCA when compared with placebo. Exercise challenge did not alter levels of plasma histamine or of LTB4 or LTC4.