Five volunteers, all with a history of childhood asthma that had persisted, though at a reduced severity and incidence, through their teens to the present, performed six treadmill runs on separate days for 6 min at 80% of VO2 max followed by 20 min of recovery. Two trials were completed under each of the following conditions: normal, positive end-expiratory pressure breathing (PEEP) during the work, and PEEP post-exercise. For each trial, forced vital capacity maneuvers (FVC) were completed twice at rest, at the 3rd and 6th min of exercise, and every 5 min during the 20-min recovery. No significant effects of PEEP in resting pulmonary function values was found nor was PET CO2 altered for rest, exercise, or recovery for PEEP vs normal trials. For the normal exercise tests, post-exercise broncho-constriction was shown by at 30% to 50% decrease in FEV1.0, 30% to 40% drop in PEF, and a 45% to 65% decrease in maximal flow at 50% VC (MEF50), comparing post-exercise values to rest. PEEP during and PEEP post conditions significantly (P less than 0.01) decreased the severity of exercise-induced asthma (EIA) as PEFR and FEV1.0 values remained within 20% of rest levels and MEF50 within 30%. Since PET CO2 was not altered by the PEEP conditions, the airway response cannot be mediated through CO2 effect, It is known that PEEP does reduce air trapping, but since PEEP during work had a lasting effect into recovery this indicates that some additional mechanism may be involved.