Nine patients with extrinsic bronchial asthma were challenged with increasing doses of histamine until there was a 20% fall in the forced expiratory volume in one second (FEV1). In association with the reduction in FEV1 there was a rise in the arterial oxygen blood tension except in three patients who were either taking steroids or cromoglycate. This rise was terminated by the administration of a bronchodilator aerosol.