Eastern white pines (Pinus strobus L.) in their 3rd year of growth photoassimilated 35, 110, 220, or 400 microcuries of (14)CO(2) on a single occasion in the spring when needle expansion was occurring. Once assimilated into organic products and translocated to various sinks, the retained (14)C represented an internal chronic source of ionizing radiation to the plant. About 2.5 months later, the pines were harvested, and the distribution of (14)C activity was determined. In addition, new needle lengths, fresh weights, and rates of processes involving CO(2) exchange were measured.All parameters measured were affected in those pines which initially assimilated 220 or 400 microcuries of (14)CO(2), whereas no significant differences were observed between control plants and those initially assimilating 35 microcuries. Plants incorporating 110 microcuries were intermediate in their responses. The percentage of assimilated (14)C retained by the plants decreased as the initial activity of assimilated (14)C increased, with the losses of (14)C occurring through both respiration and the shedding of needles. The pines which assimilated 35 microcuries of (14)CO(2) retained about 70% of the (14)C; those which assimilated 400 microcuries retained only about 28%. The distribution of the (14)C activity recovered from all the plants was essentially the same; about 65% of the total (14)C recovered was in the new needles and 13% in the roots.