The greatest sources of CH₄ are fossil fuel burning and wetlands. As one of the biggest wetland areas in the world, the Amazon region has an important role in current scenario since recent estimations indicate the CH₄ flux in the Amazon basin is approximately 22 TgC yr^-1. However, not only flooded areas but also upland forests in the Amazon could contribute as source of CH₄, with emissions ranging from 4 to 38 TgCH₄ yr^-1. Here we present preliminary results of the stable isotopic composition of CH₄ in tropical forest. This first campaign was done at Flona do Tapajós in Santarém-Para. Atmospheric air samples were collected in a vertical profile (0.2, 7, 22, 35, and 45m) in different times of the day (4, 16, 22, and 24h) during 3 days. The air samples were pulled through tubes by a battery-operated pump and then stored into glass flasks. The δ¹³C-CH₄ was determined by mass spectrometry (Carlo Erba NA 1600 equipped with Finnigan MAT Conflo interface). The δ¹³C-CH₄ varied from -47.1‰ to -48.2‰ and the overall mean value found was equal to -47.7 ±0.3‰; it is in the range for values found for atmospheric methane. Although we did not find differences among the sampling times along the day, the midnight samples were the lightest group found. Significant difference was found among the different heights, the lowest height (0.2m) showed the lightest values of δ¹³C-CH₄. The CH₄ produced during nighttime ours tends to be more depleted in ¹³C than that produced during daytime, indicating that microbial production within the canopy might be an important source of this gas. The lightly heavier and much more variable values observed at the intermediate heights, suggest that sources with different origins could co-exist at these heights and could contribute differently to the isotopic composition of the CH₄ released.