Natural cores of vegetation and soils of arctic tundra were collected in frozen condition in winter near Barrow, Alaska (71°20'N). These cores were used as microcosms in a phytotron experiment to measure the interactions, if any, between increasing atmospheric CO2 concentration and fertilization by ammonium nitrate on net ecosystem CO2 exchange and net yield of tundra vegetation. Increased soil N significantly enhanced net ecosystem CO2 uptake. The effect of increased CO2 concentration had little or no effect on mean net ecosystem carbon balance of the tundra microcosms. Added N significantly increased leaf area and phytomass of vascular plants in the microcosms while increased atmospheric CO2 had no effect on these parameters. We conclude that atmospheric CO2 is not now limiting net ecosystem production in the tundra and that its direct effects will be slight even at double the present concentration. the most probable effects of carbon dioxide in the coastal tundra will be through its indirect effects on temperature, water table, peat decomposition, and the availability of soil nutrients.