Nonspecific bronchial reactivity to exercise and histamine was measured in nine children with asthma before and within nine days after allergen inhalation. All patients developed an immediate fall in FEV1 of ≥16% after allergen inhalation, and five children also developed a definite late asthmatic response with a fall in peak expiratory flow of ≥14% after 3 to 8 hours. Mean postexercise fall in FEV1 (ΔFEV1) of the whole group was significantly greater after allergen challenge compared with that of control subjects (29 ± 6% and 16 ± 4%; p = 0.013). There was no change in refractoriness to repeated exercise after allergen challenge. The mean provocation concentration of histamine causing a decrease in FEV1 of 20% of the whole group was less after allergen challenge compared with that of control subjects (0.47 ± 0.18 and 0.62 ± 0.13), but this was not statistically significant (p = 0.19). Of the five children with late allergen reactions, all demonstrated increased histamine sensitivity, and all four children who developed definite exercise-induced asthma also demonstrated increased sensitivity to exercise. Of the four children without late allergen reactions, none demonstrated increased histamine sensitivity, but two of the three children with definite exercise-induced asthma demonstrated increased sensitivity to exercise. It may be that sensitivity to exercise is more easily affected by nonspecific reactivity than sensitivity to histamine. It is concluded that increased bronchial responsiveness to both exercise and histamine occur after allergen provocation in patients with asthma.