The relation between exercise-induced airway obstruction and changes in the gas exchange function of the lung was studied in 6 subjects with asthma. Continuous distributions of ventilation perfusion (VA/Q) ratios and spirometry were measured before and at intervals after an exercise challenge that caused a greater than 20% decrease in FEV1 or in peak expiratory flow rate (PEFR). Before exercise, all subjects had unimodal VA/Q distributions that were broader than normal (mean log SD of blood flow, 0.54) and all developed wider distributions 15 to 28 min after the start of the 8-min exercise challenge (mean log SD of blood flow, 1.02). Two subjects developed bimodal VA/Q distributions, but all distributions returned to baseline configuration within 58 min, usually before FEV1 and PEFR had returned to baseline values. There was no relation between the decrease in FEV1 or PEFR and the degree of VA/Q inequality. Aerosolized salbutamol resulted in improved spirometry and further narrowing of the VA/Q distributions in 3 subjects tested.