The capacity of salbutamol 0.3 mg (SAL), disodium cromoglycate 3 mg (DSCG), a combination (SAL + DSCG), and ipratropium bromide 80 ug (IB), all given as metered aerosols to prevent exercise-induced (EIA), was compared with that of a placebo, a peroral lactose pill. Seven children participated, having reproducible EIA provoked by running on a treadmill at a heart rate of 170 sustained for 6 min. FEV1 and volume of trapped gas (VTG), defined as the air volume released during rebreathing oxygen with maximum breaths at the end of a multiple breath nitrogen wash out, were used as tests of spasm in large and small airways. SAL and SAL + DSCG offered complete protection in large and small airways. DSCG and IB prevented EIA in large airways (FEV1) to 95%, but only to about 50% in small airways (VTG). SAL or SAL + DSCG gave significantly better protection (FEV1 and VTG) than DSCG and IB (P less than 0.01). Differences between DSCG and IB were not statistically significant. DSCG or propellant caused significant irritation and spasm in small airways (VTG) before exercise. Most subjects seemed to obtain satisfactory protection against EIA by beta 2-agonists.