Longitudinal variations in bronchial responsiveness to methacholine was studied in 19 male crosscountry skiers 19–21 years old during the season of their sport and compared with the response in 22 age-matched control subjects with minimal physical activity. The methacholine concentration required for a 10% fall in the forced expiratory volume in the first second (PC10) remained stable from August to November in the group of skiers, then decreased from November to February and finally increased from February to June. In the control group, PC10 increased from August to November, increased further to February and finally decreased from February to June. Alternative methods for expressing bronchial responsiveness as the linear dosc-response slope or the logarithmic dose-response slope gave no more information than did PC10 alone. High correlations were found between PC10 and either of the dose-response slopes. PC10 and the dose response-slopes were equally reproducible when measured on two successive days. In contrary to the control group, high-performance cross- country skiers demonstrated increased bronchial responsiveness in the winter. The PC10 was an advantageous method for studying longitudinal changes in bronchial responsiveness among healthy subjects.