The relationship between the effect of deep inspiration on Vmax and basal bronchomotor tone was studied by partial and maximum expiratory flow-volume curve in 16 young healthy females (20-21 years old). Effect of deep inspiration on Vmax (DI index; (PEF25-MEF25)/PEF25) significantly related to percent increase in PEF25 not only by inhalation of ipratropium bromide (r = 0.81, p less than 0.0002) but also by inhalation of salbutamol (r = -0.62, p less than 0.01). Furthermore, day to day variation of DI index significantly related to day to day variation of PEF25 (r = 0.68, p less than 0.005) but not to that of MEF25. These findings suggest that the bronchodilating effect of deep inspiration in young healthy females may depend on intensity of basal bronchomotor tone caused by tonic vagal nerve activity.