Export and photosynthesis i n leaves of Salvia splendens were measured concurrently during steady-state 14C0, labeling conditions. Under ambient CO, and O, conditions, photosynthesis and export rates were similar at 15 and 25"C, but both declined as leaf temperature was raised from 25 to 40°C. Suppressing photorespiration between 15 and 40°C by manipulating CO, and O, levels resulted in higher rates of leaf photosynthesis, total sugar synthesis, and export. There was a linear relationship between the rate of photosynthesis and the rate of export between 15 and 35°C. At these temperatures, 60 to 80% of the carbon fixed was readily exported with sucrose, raffinose, and stachyose, which together constituted over 90% of phloem mobile assimilates. Above 35"C, however, export during photosynthesis was inhibited both in photorespiratory conditions, which inhibited photosynthesis, and in nonphotorespiratory conditions, which did not inhibit photosynthesis. Sucrose and raffinose but not stachyose accumulated i n the leaf at 40°C. When leaves were preincubated at 40°C and then cooled to 3 5 T , export recovered more slowly than photosynthesis. These data are consistent with the view that impairment of export processes, rather than photosynthetic processes associated with light trapping, carbon reduction, and sucrose synthesis, accounted for the marked reduction i n export between 35 and 40°C. Taken together, the data indicated that temperature changes between 15 and 40°C had two effects on photosynthesis and concurrent export. At all temperatures, suppressing photorespiration increased both photosynthesis and export, but above 35"C, export processes were more directly inhibited independent of changes i n photorespiration and photosynthesis.