This study looked at seasonal fluctuations of airway responsiveness (AR) to methacholine and exercise in ten mild asthmatic and seven normal subjects. Each subject had a monthly methacholine inhalation test. An exercise challenge with measurement of expiratory flows was performed initially in the fall (F), then in winter (W), and in summer (S). Throughout the study, the subjects were asked to record on a diary card twice daily peak flow rates and respiratory symptoms, one week a month. Airway responsiveness to exercise and methacholine remained generally stable throughout the year, although an increase in respiratory symptoms occurred during fall and winter. The overall AR to methacholine was not significantly different during the different seasons (F, W, S and Spring) with the methacholine concentration producing a 20% fall in FEV1, PC20 (mg/ml) values, respectively, of 1.7 +/- 1.2 mg/ml, 1.8 +/- 1.1, 2.1 +/- 1.2, and 2.0 +/- 1.9 for asthmatics and 79.0 +/- 1.2 mg/ml, 66.8 +/- 1.0, 87.3 +/- 1.0, and 73.1 +/- 1.0 for normals. However, short term variations in AR were associated to exposure to antigens and cold weather. Mean daily peak expiratory flows remained generally stable through the seasons. On the three exercise tests, the VO2 max and the mean % fall in FEV1 after maximal exercise showed large variations; these, however, were not significantly different (mean fall: 16.2% (F), 16.6% (W), and 14.7% (S) in asthmatics). In conclusion, although it may increase transiently, overall airway responsiveness to methacholine and exercise remains generally stable in asthmatic and normal subjects throughout the year.