The physiological responses of 6- to 8-year-old aspen (Populus tremuloides Michx.) stems to temperature, light, and CO(2) concentration were investigated in the field throughout the year using infrared CO(2) analysis. Light response studies showed that the rate of gross photosynthesis was linear from 0 to 400 ft-c (0 to 1.6 mw/cm(2) of 400-700 nm) with light saturation being reached between 800 to 1400 ft-c (3.2 to 5.6 mw/cm(2) of 400-700 nm). At this light intensity, the respiratory CO(2) loss was reduced to 10 to 15% of dark rates. Net photosynthetic CO(2) uptake was not observed even at intensities as high as 3400 ft-c (13.6 mw/cm(2) of 400-700 nm). The light response curve was similar for both winter and summer stems.During summer months, the respiratory and photosynthetic rates of the aspen stem increased with temperature at a near constant rate between 5 and 35 C. For winter stems, the gross photosynthetic rate increased in a pattern similar to the dark respiratory rate as the temperature rose from 3 to 17 C. Below 0 C and above 17 C, however, the gross photosynthetic rate fell off in relation to the respiratory rate so that the per cent of CO(2) reassimilated decreased from 75% to less than 50%. Measurable bark photosynthetic activity was not observed below -3 C.The gross photosynthetic rate of stems was not affected when the gas passing through the cuvette contained concentrations of CO(2) ranging from 0 to 580 mul CO(2)/l air.