The relations between CO2 uptake, translocation, and carbon accumulation in several vegetative components of Douglas fir seedlings (Pseudotsuga menziesii [Mirb.] Franco) have been quantified using 14CO2. Seedlings were exposed to a constant specific radioactivity of 14CO2 and a repeating daily pattern of temperature and light for 4 consecutive days. Results of 14C analysis, which indicated a transitory pattern of photoassimilated carbon movement, were extrapolated to a “steady rate” using a compartment analysis. Accumulation rates of photoassimilated carbon, relative to tissue carbon, were new needles, 0.94%/day, old needles, 1.14%/day, new shoots 0.38%/day, stem, 0.16%/day, and roots, 0.50%/day. Therefore, the source of carbon, the needles, is also the strongest sink.