In an attempt to elucidate the roles of prostaglandin F2alpha and noradrenaline in exercise-induced asthma, plasma levels of these substances were measured at rest, during and after exercise in normal and asthmatic subjects. Changes in airways resistance were assessed by measuring the peak expiratory flow rate. Plasma levels of 15-keto-13,14-dihydro-prostaglandin F2alpha (a relatively stable metabolite of prostaglandin F2alpha) and noradrenaline, at rest, and the changes that occurred during and after exercise, were similar in both the normal and asthmatic subjects. Exercise-induced broncho-constriction occurred in all asthmatic subjects following exercise while no significant change occurred in the peak expiratory flow rates of the normal subjects. At this time, in both groups of subjects, the levels of 15-keto-13,14-dihydro-prostaglandin F2alpha were lower than at rest. The level of noradrenaline observed following exercise were lower than during exercise in all but one subject. It would appear that neither noradrenaline nor prostaglandin F2alpha (or its metabolites) play a significant role in the aetiology of exercise-induced asthma.