Closed flow chambers are commonly used to measure CH4 emissions from aquatic plants. We monitored environmental and plant physiological responses to closed chambers during measurements of CH4 emission from Typha latifolia (cattails) to evaluate potential errors in emission estimates. In 1990, two wetlands were studied: one in NE Kansas and one in central New York. Leaves and stems of Typha enclosed in gas-impermable, collapsible bags were exposed to rapid increases in chamber air temperature and, thus, tissue temperatures (up to 9oC above ambient) as well as rapid reductions in both CO2 partial pressure due to photosynthesis and the leaf-to-air water vapor pressure deficit due to transpiration. These rapid (< 5 min) environmental changes within chambers are known to affect stomatal aperture through which CH4 exists Typha leaves. Indeed, stomatal conductance (g) increased up to 42% compared to control leaves after 19 min of enclosure in chambers, and chamber effects on interactions between irradiance and g were detected. The duration of enclosure also critically affected the CH4 gradient within chambers. Estimated CH4 emission from leaves in chambers sampled after 19 min (3 nmol m-2s-1) was 10-fold lower than estimates based on 30-s sampling periods (30 nmol m-2s-1) due to reduction in the leaf-to-air concentration gradient for CH4. Since closed chambers 1)induce alterations in g, 2)may result in rapid environmental changes, and 3)lead to almost instantaneous reductions in leaf-to-air concentration gradient for CH4 diffusion, we suggest that only very short enclosure times should be used when measuring CH4 emission from plants