Peatlands are generally small sinks for atmospheric CO2. However, the sustainability of this sink functioning is threatened in a changing climate. We measured the M exchange in a temperate bog between August 2005 and July 2006 using the eddy covariance technique. During this period, the CO2 balance was -78.6 +/- 20.0 g CO2 m(-2) yr(-1), which is a lower uptake than others have reported for comparable ecosystems, but in accordance with average Holocene uptake rates. Average winter emissions were small (11 mu g CO2 m(-2) s(-1)). but the accumulated non-growing season fluxes (121 g CO2 m(-2)) represent a larger flux than the annual balance. Taking into consideration that wintertime temperatures in the northern hemisphere are expected to increase in the future, wintertime fluxes may become increasingly important. The highest instantaneous monthly average uptake occurred in June (-38 mu g CO2 m(-2) S-1), while the drier month of July showed a distinctly smaller sink (-22 mu g CO2 m(-2) s(-1)). It is suggested that soil drying caused gross primary production to decline. while nighttime respiration showed a steadily increase. Our findings indicate that the effect of future warmer temperatures on the growing season CO) exchange in peatlands is dependent on soil wetness.