To assess the variability of net photosynthetic CO(2) exchange per unit leaf area and to construct budgets for stands of field-grown tobacco (Nicotiana tabacum, Connecticut Broadleaf), a number of short-time measurements were made on all available leaf positions on two varieties using a hand-held transparent chamber for conducting gas exchange measurements on leaves. Measurements of net CO(2) exchange were carried out on 18 separate days during a 35-day period, beginning 22 days after the seedlings were transplanted to the field. Gas exchange assays on leaves were conducted under ambient conditions of temperature and light intensity at all times of day. Solar radiation was monitored throughout the period, and losses of respiratory CO(2) from stems, roots, and leaves (in the dark) were estimated. A simple model was proposed to relate daily total CO(2) input to irradiance and total leaf area. The total leaf area was assumed to be a function of day number. Dark respiratory losses accounted for 41% to 47% of total CO(2) assimilation. Analysis of variance indicated that the two varieties were not significantly different in whole plant rate of CO(2) fixation per unit of leaf area. CO(2) input was closely associated with leaf area within each variety. Throughout the experiment, the difference between the two varieties in total leaf area per plant was the largest single factor in determining net CO(2) inputs. The cumulative dry weight increase for each variety was similar to the prediction of net dry matter input obtained by gas exchange measurements, thus confirming the close relationship between total plant net CO(2) assimilation and dry weight yield.