Hyperventilation in asthmatic subjects causes airways obstruction as demonstrated by a fall in the Forced Expiratory Volume in one second. Clinically, measurements of blood gases during asthmatic attacks have shown that asthmatic subjects, who hyperventilate have a blood carbon dioxide level below normal unless ventilatory failure occurs. Two methods of demonstrating that asthmatic subjects hyperventilate on the thought of asthma were employed. 1. Chronic asthmatic subjects had their respiratory minute volume measured during relaxation and at the recall of an asthmatic attack. The mean minute volume rose from 10.1 litres when relaxed to 13.5 litres when recalling an asthmatic attack (p < .025). 2. Asthmatic volunteers had their respiratory minute volume measured during relaxation and on the hypnotic suggestion of anger, fright and asthma and pleasurable excitement, both in the conscious state and under hypnosis. The data were statistically treated by an analysis of variance. All emotion resulted in a significant increase in minute volume at the 0.1 level of significance.