To explore the physiological mechanisms underlying ozone-induced growth reductions in loblolly pine (Pinus taeda L.), seedlings were exposed to sub-ambient (charcoal-filtered), ambient or twice-ambient ozone in open-top chambers for three growing seasons. In the final year of exposure, current-year needle fascicles were labeled with (CO2)-C-14 and the incorporation of C-14 into biochemical fractions was followed for 48 hours. Irrespective of ozone treatment, losses of C-14-assimilates from foliage to respiration and translocation were minimal during the first 3 hours, whereas more than 60% of the label was lost during the next 45 hours. Radiolabel in sugar decreased rapidly after a lag period, roughly paralleling the pattern of total C-14 loss. The amount of C-14 label in starch and lipids plus pigments remained constant throughout the 48-hour chase period, whereas the amount of C-14 label in other fractions showed a net decrease over the 48-hour chase period. Ozone treatments altered foliar carbon dynamics in two ways: (1) ozone exposure increased foliar C-14 retention up to 21% for the first 5 hours after labeling, but not thereafter, and (2) ozone exposure decreased partitioning of C-14 into starch and increased partitioning of C-14 into organic acids, residue, and lipids plus pigments, indicating an intensified partitioning of carbon to injury and repair processes. Both short-term carbon retention and diversion of carbon from storage compounds to repair processes are foliar mechanisms by which ozone exposure could decrease growth in loblolly pine seedlings.