Recent evidence indicates that significant amounts of C may be lost as CO2 to the atmosphere from tundra ecosystems during the fall, winter and spring months. Because high latitude ecosystems are particularly vulnerable to climate change, and contain large soil C stocks, the annual C budget is of particular interest. Significant amounts of CO2 loss were observed in arctic tundra ecosystems of the North Slope of Alaska during the 1993–1994 cold season. Moist tussock tundra ecosystems lost approximately 0.3 gC m−2 d−1 between mid-October and late May while coastal wet sedge ecosystems lost on average 0.08 gC m−2 d−1. Efflux rates were greatest during the months of October and May when soil temperatures were at a maximum, and portions of the soil profile were unfrozen. These daily loss rates equate to a seasonal loss of roughly 70 and 20 gC m−2 season−1 for tussock and wet sedge tundra ecosystems, respectively. Combined with warm season estimates of net CO2 efflux, tussock tundra ecosystems were net sources of approximately 112 gC m−2 yr−1 (0.1 PgC yr−1 worldwide) over the 1993–1994 calendar year, while wet sedge ecosystems lost approximately 25 gC m−2 yr−1 (0.02 PgC yr−1worldwide). This study indicates that estimates of annual net CO2 exchange, based on warm season measurements alone, underestimate the actual magnitude of CO2 efflux.