Bronchial responsiveness to inhaled histamine and exercise was measured in 19 asthmatics. Histamine aerosol was inhaled to determine the provocative concentration producing a 20% fall in forced expired volume in one second (FEV1) (PC20). Exercise was performed on a treadmill and a cycle ergometer; following each procedure the percent fall in the FEV1 (delta FEV1) and the exercise lability (percent rise in FEV1 plus percent fall in FEV1) were calculated. Delta FEV1 and exercise lability after both forms of exercise were similar. PC20 correlated with delta FEV1 and exercise lability in both forms of exercise; however, the correlation with exercise lability was better. PC20 was more sensitive in demonstrating bronchial hyperresponsiveness. The close correlation between the level of bronchial responsiveness to histamine and exercise supports the view that release of endogenous chemical mediators is an important determinant of exercise-induced asthma. The treadmill exercise and cycle ergometry protocols were equally effective in producing exercise-induced asthma.