These studies were undertaken to elucidate the reasons for the low productivity of ginseng by comparing the rate of CO_2-fixation and photorespiration, variation in the amounts of intermediates and enzyme activities of glycolate oxidase and catalase in ginseng with those of potato. The ability of CO_2-fixation in ginseng was found to be one half of that of potato and there were significant differences between those two plants in the rate of ^14C incorporated into glutamate, aspartate, malate and 3-PGA, but little differences in P-glycolate, glycolate, serine and glycine. The ratio of photorespiration to dark respiration and the activities of glycolate oxidase and catalase in the two species were about same, but ginseng showed higher ratio in photorespiration to total CO_2-fixation than potato did. These results indicated that the low productivity of ginseng may resulted from the low CO_2-fixation ability and high rate of photorespiration.