Measurements of gaseous ammonia above a montane-subalpine forest in the Colorado mountains show that the role of the forest as a source or sink depends on the atmospheric concentrations. The canopy appeared to be an ammonia source when exposed to air containing low concentrations, but a sink when exposed to air enriched by nearby agricultural sources. The forest-averaged compensation point was 0.8 part per billion by volume at 20 degrees C. The net burden of ammonia and other nitrogen species of anthropogenic origin at this site was much less than at forest sites in the eastern United States and Europe and may provide a valuable resource for this nitrogen-limited ecosystem.