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International audience; Terrestrial ecosystems are significant sources of atmospheric gases such as reactive nitrogen (NH3, NO and N2O). Conversely, they can be a sink for NH3, NOx, and O3, which can lead to several adverse effects (acidification and eutrophication, direct effects). But deposition also contributes to decreasing their lifetime in the atmosphere, and is therefore an efficient “depollution process”. In this context, continuous measurements of O3 fluxes as well as other compounds have been set up at two sites in France. The fluxes of O3, CO2/H2O are measured continuously with the eddy covariance method over the “LeBray” pine forest near Bordeaux since 2003, and over a cropland field near Paris since mid-2004. The Bray forest site is 35 year old plantation of maritime pine (Pinus pinaster Ait.) with an understorey of graminae. It is currently a CarboEurope site and has been part of the Euroflux network since 1996. The trees are distributed in parallel rows along the NE-SW axis. The inter row distance is 4m and the stand density is about 500 trees ha-1. The mean tree height is 20 m and the LAI around 2.6. The site is flat and the fetch is over 600 m in the prevailing wind direction. The climate is oceanic with 900 mm of rainfall per year. Instruments are installed at 43 m. The cropland site is a CarboEurope site, with a rotation of mustard, maize, wheat and barley, with a nitrogen supply of 120 kg ha-1 y-1, as mineral fertiliser, and an additional 100 kg ha-1 every three years as cattle slurry. The climate is continental-oceanic, with 600 mm of rainfall per year, the soil is a luvisol with 60% of silt, and 30% of clay. Instruments are installed at 2.5 to 4.5 m. At both sites, the eddy-covariance equipment set includes a Gill RGA3-50 or a Gill R2 sonic anemometer, a H20 and CO2 Licor 7500 analyser, and a fast O3 chemiluminescent based close-path sensor (ATDD V2.0) with solid-plates “targets” coated with Coumarin. Additional absolute measurements are performed at both sites. Automatic chambers measuring N2O and NO emission will be installed in 2005 in Grignon, as well as NH3 gradient measurements. Initial results from these two sites will be presented.