1. The protective effect of different particle sizes of sodium cromoglycate (SCG) was assessed by exercise challenge in nine asthmatic patients.
2. SCG particles of 11·7 μm or 2·0 μm mass median diameter were generated by a spinning disc and compared with lactose placebo particles. The drugs were given 10 min before a 6 min run on a treadmill.
3. The post-exercise bronchoconstriction caused a 48% fall in peak flow rate in the placebo test, a 41% fall after the large SCG particles and a 20% fall after the small particles.
4. It is concluded that the site of action of SCG is probably in the smallest airways.