1. Immediately after breath-holding at end-expiratory level, there is a certain period of no particular respiratory sensation which is terminated by the onset of an unpleasant sensation and followed by progressive discomfort during breath-holding, This period, defined as the time from the start of voluntary breath-holding to the point where the onset of an unpleasant sensation occurs, is designated 'the period of no respiratory sensation', Although it has been shown that the maximum breath-holding performance is improved with successive trials, it is not clear whether this training effect exerts a similar influence on the period of no respiratory sensation during breath-holding, 2. Since the training effect seems to be associated with the stresses of breath-holding, we hypothesized that the initial period of no respiratory sensation during breath-holding might be less influenced by the training effect, 3. We studied 13 normal subjects who performed repeated breath holds while continuously rating their respiratory discomfort using a visual analogue scale, In addition, we measured the hypercapnic ventilatory response of each individual and obtained the relationship between the slope of the hypercapnic response curve and breath-holding periods, 4. Our results showed that there was little training effect on the period of no respiratory sensation and that the period of no sensation during breath-holding is inversely related to the slope of the hypercapnic ventilatory response curve, 5. The period of no respiratory sensation was also measured in eight patients with chronic obstructive pulmonary disease, The values of the period of no respiratory sensation in patients,vith chronic obstructive pulmonary disease were apparently lower than those obtained in normal subjects, 6. These findings suggest that measurement of the period of no respiratory sensation can be a useful clinical test for the study of genesis of dyspnoea.
