The effect of intravenous infusion of dipyridamole on histamine- and adenosine-induced bronchoconstriction was studied in 6 normal and 6 allergic asthmatic subjects. Dipyridamole in a single dose of 50 mg over 50 min had no effect upon baseline airway calibre measured as specific airways conductance (sGaw) and the forced expiratory volume in 1 s (FEV1). In the asthmatic subjects adenosine-induced bronchoconstriction was potentiated to a small extent by dipyridamole as indicated by a decrease in the concentration of adenosine required to produce falls from baseline of 40% for sGaw (PCs40) and 20% for FEV (PCf20) from 0.48 mg/ml to 0.32 and 0.37 mg/ml respectively. Dipyridamole had a small protective effect on histamine-induced bronchoconstriction with PCs40 and PCf20 increasing from 0.08 mg/ml to 0.11 and 0.17 mg/ml. In normal subjects dipyridamole failed to reveal a bronchoconstrictor effect of adenosine. These studies demonstrate the complex interaction between dipyridamole and bronchoconstrictor stimuli on the airways.