The bronchial response to cycling and running was compared in six adult asthmatic persons. The effects of different air conditions during cycling regarding the induction of bronchoconstriction was studied. The exercise consisted of 6 minutes' work at an intensity of 80-85% of maximal heart rate. Heart rate, oxygen consumption and ventilation were measured to check that the exercise level was the same in all tests. Peak expiratory flow (PEF) was used to test for bronchoconstriction. Bicycling and treadmill running were performed under indoor conditions and bicycling while breathing cold, dry air (-18 degrees C) and room-tempered humid air (60% RH), respectively. No difference in bronchoconstriction was found between cycling and running under indoor conditions. However, bicycling exercise with inhalation of cold dry air provoked more bronchoconstriction than when inhalating humid air (PEF reductions of 19.4+/-6% and 6.1+/-2%, respectively). No differences were found between the exercise modes in heart rate, oxygen consumption, ventilation per minute, respiratory rate, carbon dioxide elimination or subjective ratings of perceived exertion and breathlessness. It is concluded that it is not the type of exercise, but the ventilation demand and humidity of the inspired air that are the main determinants of the occurrence and degree of bronchoconstriction.