Although some CO(2) released by respiring cells in tree stems diffuses directly to the atmosphere, on a daily basis 15-55% can remain within the tree. High concentrations of CO(2) build up in stems because of barriers to diffusion in the inner bark and xylem. In contrast with atmospheric [CO(2)] of c. 0.04%, the [CO(2)] in tree stems is often between 3 and 10%, and sometimes exceeds 20%. The [CO(2)] in stems varies diurnally and seasonally. Some respired CO(2) remaining in the stem dissolves in xylem sap and is transported toward the leaves. A portion can be fixed by photosynthetic cells in woody tissues, and a portion diffuses out of the stem into the atmosphere remote from the site of origin. It is now evident that measurements of CO(2) efflux to the atmosphere, which have been commonly used to estimate the rate of woody tissue respiration, do not adequately account for the internal fluxes of CO(2). New approaches to quantify both internal and external fluxes of CO(2) have been developed to estimate the rate of woody tissue respiration. A more complete assessment of internal fluxes of CO(2) in stems will improve our understanding of the carbon balance of trees.