The immediate changes in regional ventilation and pulmonary blood flow were studied in seventeen adults with perennial asthma and in two control persons, who were challenged by histamine inhalation (histamine induced asthma (HIA)). The studies were based on gammacamera images of 81mKr and of 99mTc labelled particles in the lungs and the results were compared with simultaneous changes in peak expiratory flow (PEF). No changes were observed in normal persons. After histamine inhalation the ventilation pattern became patchy, but no changes were seen in the blood flow distribution. PEF was markedly reduced. On repeated challenges ventilatory changes after HIA did not result in stereotypic images. HIA-defects were dissimilar to exercise- and antigen-induced defects when studied in the same patient(s) and were different, too, from the result in the same patients during a spontaneous asthmatic attack. The therapeutic and prophylactic effects of terbutaline were demonstrated in patients exposed to a histamine challenge.