We used the eddy—correlation technique to investigate the exchange of CO2 between an undisturbed old—growth forest and the atmosphere at a remote Southern Hemisphere site on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO2 exchange, and to test whether present knowledge of leaf—level processes was sufficient to understand ecosystem—level exchange. On clear summer days the maximum rate of net ecosystem CO2 uptake exceeded 15 μmol°m—2°s—1, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between °—2 and —7 μmol°m—2°s—1. Nighttime CO2 efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem CO2 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature, saturation deficit, and the diffuse PPFD were of lesser, but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However, at night, the short—term exchange of CO2 between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between nocturnal CO2 exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO2 exchange. The daytime light—use efficiency of the ecosystem (CO2 uptake/incident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/mol after frosts on clear winter days. Ecosystem CO2 uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.