1. The changes in peak expiratory flow rate (PEF) and plasma cortisol were studied in relation to a 6 min period of treadmill running in six normal and eighteen asthmatic subjects. Of the asthmatics patients, five were not receiving treatment with steroids, six were receiving low doses of steroids (under 7·5 mg of prednisone daily) and seven were receiving high doses of steroids (over 7·5 mg of prednisone daily) at the time of study.
2. All subjects were studied twice within 1 week at similar times of day, once after premedication with sodium cromoglycate (SCG) and once after a placebo.
3. Resting PEF and plasma cortisol did not differ between placebo and SCG tests.
4. No change in PEF occurred as a result of exercise in the control subjects. The asthmatic patients developed post-exercise bronchoconstriction which was partly prevented by SCG but was not affected by steroids.
5. Plasma cortisol rose after exercise in the asthmatic subjects but not in the control subjects. The rise may have been related to the stress of exercise-induced asthma. SCG had no significant effect on plasma cortisol after exercise.