Monthly estimates of gross primary production (gross uptake of CO2 by plants) and ecosystem respiration (gross release of CO2 from the ecosystem) in an oak-pine forest in the northeastern United States were used in this study to examine the types of metabolic changes in terrestrial systems that might yield the increased seasonal amplitude of CO2 concentrations observed at several monitoring stations in recent years. In this study, increases in either photosynthesis or respiration increased the amplitude of the seasonal oscillation of CO2 concentrations if the increases were predominantly in the northern hemispheric summer and winter, respectively. The quantitative changes in metabolism required to produce the observed increase in amplitude, however, were too large to be explained by CO2 fertilization or by a temperature-induced increase in winter respiration. Investigations of the role of the biota in causing seasonal and year-to-year variations in atmospheric CO2 concentrations are limited by the lack of stations monitoring CO2 in continental air. ¿American Geophysical Union 1987