We report on net ecosystem CO2 exchange (NEE) measurements conducted with the eddy covariance method over agricultural peat soil in the 2-year period between October 2000 and October 2002. In 2001, spring barley and undersown grass were sown on the site. After the barley harvest, the perennial forage grass was left to grow, so that in 2002 the field was growing grass. A higher maximum net CO2 uptake was observed for barley than for grass during the height of the summer, peaking at about -1.0 and -0.75 mg CO2 m-2 s-1, respectively. The maximum nighttime total ecosystem respiration was measured in July and was similar for both crops, about 0.35 mg CO2 m-2s-1. During the growing season the field acted as a daily CO2 sink for only 40 days in barley versus 84 days in grass. In the winter the average carbon dioxide efflux varied from 15.6 to 16.5 ¿g CO2 m-2 s-1. The annual NEE of the agricultural peat soil growing barley and grass was 771 ¿ 104 and 290 ¿ 91 g CO2 m-2, respectively. The longer net CO2 uptake period was the main reason for the lower annual NEE for grass; however, owing to the higher amount of grass biomass produced the net ecosystem production (NEP), calculated as the sum of NEE and removed biomass, was slightly larger for grass (452 g C m-2) than for barley (336 g C m-2). These results show that the organic peat is still undergoing rapid decomposition after more than 100 years of cultivation activity. In addition, switching from an annual to a perennial crop did not turn the field into a CO2 sink, at least during a 1-year period.