Carbon dioxide (CO2) fluxes and grass production were measured in adjacent new and permanent intensively managed, Lolium perenne grasslands. Cumulative grass production during 2004 was 0.38 and 0.70 kg C m(-2) year(-1) in the new and permanent sites, respectively. Chamber measurements were used to create site-specific statistical response functions of gross primary production (GPP) and total respiration (R-TOT) at the two sites. A 1-year time series of GPP and RTOT was reconstructed by combining these functions with measurements of leaf area index (LAI) and continuous meteorological data. Net ecosystem exchange (NEE) was estimated by subtracting RTOT from GPR Seasonal variation in GPP closely followed the changes in grass production and LAI. RTOT increased in summertime due to increased soil temperature and optimal soil moisture conditions. Both sites were net CO2 Sinks during the grass growing season. Cumulative values of GPP, RTOT and NEE in the new grassland were 2.14, -1.98 and 0.15 kg C m(-2) year(-1), respectively. In the permanent grassland cumulative values of GPP, RTOT and NEE were 2.90, -2.52 and 0.38 kg C m(-2) year(-1), respectively. (c) 2005 Elsevier B.V. All rights reserved.