CO2 released by respiring cells in tree stems can either diffuse to the atmosphere or dissolve in xylem sap. In this study, the internal and external fluxes of CO2 released from respiring stems of five sycamore (Platanus occidentalis L.) trees were calculated. Mean rates of stem respiration were highest in mid-afternoon and lowest at night, and were positively correlated with air temperature. Over a 24 h period, on average 34% of the CO2 released by respiring cells in the measured stem segment remained within the tree. CO2 efflux to the atmosphere consisted of similar proportions of CO2 derived from local respiring cells (55%) and CO2 that had been transported in the xylem (45%), indicating that CO2 efflux does not accurately estimate respiration. A portion of the efflux of transported CO2 appeared to have originated in the root system. A modification of the method for calculating stem respiration based on internal and external fluxes of CO2 was developed to separate efflux due to local respiration from efflux of transported CO2.