In an attempt to elucidate the roles of prostaglandin F2.alpha. [PGF2.alpha.] 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-PGF2.alpha. (a relatively stable metabolite of PGF2.alpha.) and noradrenaline, at rest, and the changes that occurred during and after exercise, were similar in both the normal and asthmatic subjects. Exercise-induced bronchoconstriction 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-PGF2.alpha. were lower than at rest. The levels of noradrenaline observed following exercise were lower than during exercise in all but 1 subject. Apparently neither noradrenaline norPGF2.alpha. (or its metabolites) play a significant role in the etiology of exercise-induced asthma.