Twenty-two subjects complaining of cough, chest tightness and/or shortness of breath with normal chest radiograph and normal pulmonary function test results were challenged with methacoline. Venous histamine levels were measured before and just after methacoline challenge. For comparison, a single blood sample was obtained from 10 normal subjects. Blood histamine levels were significantly higher in subjects with respiratory symptoms regardless of methacoline challenge being positive or negative (p less than 0.001). After methacoline challenge, blood histamine increased significantly in methacoline challenge positive group (p less than 0.05). Thereafter, terfenadine, a H1 antagonist, 120 mg/day were given to patients for one month. After terfenadine therapy, there was a subjective improvement of symptoms, methacoline provocation dose (PD20) increased and there was no significant change in blood histamine level. It is concluded that nonspecific challenge increases blood histamine levels and blood histamine levels seems to be a sensitive index of bronchial hyperreactivity in subjects with respiratory symptoms of unknown origin and that terfenadine is effective in the treatment of bronchial hyperreactivity.