Two-year-old Douglas-fir seedlings accumulated photoassimilated carbon in the tissues of new needles, previous years' needles, new shoots, stem, and roots at rates relative to tissue carbon of 1.25, 0.57, 0.42, 0.60, 0.61%/d, respectively. These daily net accumulation rates were measured just after bud set, and the rates parallel the carbon accumulation expected based on phenological observation. Results were obtained with a new system for long-term labelling of groups of seedlings with 14CO2 at a constant specific activity. This system is described.