Nitrate assimilation and photosynthetic responses of Ctentdium mulluscum (Hedw.) Mitt. Homalothecium sericeum (Hedw.) Br.Eur. Pleurusium schreberi (Brid.) Mitt. Hylocomium splendens (Hedw.) Br.Eur. were studied during and after controlled exposures to 35 nl 1−1NO2 and 35 nl 1−1NO. NO2 induced nitrate reductase activity in all species 24 hours after the start of fumigation. The magnitude of the inductions varied between 23 and 89%. In contrast, exposure to NO caused a rapid decline in activity within 24 hours. After 21 days continuous exposure to each of the gases, the plants were supplied with a single application of 10 mM, NaNO3. Nitrate reductase activity was induced only in plants exposed to clean air. NO8exposure caused a loss of nitrate inducibility of NR. There were no marked effects on rates of oxygen evolution. After a three-week pollution-free period, the bryophytes regained the capacity for nitrate assimilation.