This study was conducted in 2003 and 2004 at the km 67 old growth forest in the Tapajós National Forest (2,85o S; 54,05º W. The objective was measure the carbon isotope ratio (13C/12C) of respired CO2 from the entire ecosystem and isotope composition of organic components leaves, soil, litter and dead wood). The Keeling plot technique and Farquhar’s leaf model was used to examine the physiological drivers of the isotopic composition of these components as well as the seasonal response for them. A variation of respired d13CR - CO2 by the ecosystem was well related with precipitation variation, and a significant seasonal difference was found in 2003. The d13C of leaf organic matter showed a clear stratification along the vertical profile and significant seasonal variation at the top of canopy in 2003. Significant correlations were found between d13CR - CO2 and photosynthetically active radiation (PAR), vapor pressure deficit (VPD). The estimated ci/ca ratio values showed significant differences between heights and seasons. The results indicated that the isotopic composition of respired CO2 and organic matter was sensitive to microclimatic variations; so far the d13C values can be used to understand how environmental changes can affect the carbon cycle at ecosystem scale.