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 14CO2 and the incorporation of 14C into biochemical fractions was followed for 48 hours. Irrespective of ozone treatment, losses of 14C-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 14C loss. The amount of 14C label in starch and lipids plus pigments remained constant throughout the 48-hour chase period, whereas the amount of 14C 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 14C retention up to 21% for the first 5 hours after labeling, but not thereafter, and (2) ozone exposure decreased partitioning of 14C into starch and increased partitioning of 14C 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.