The initial products of photosynthesis by the C(3) species Flaveria cronquistii, the C(4) species F. trinervia, and the C(3)-C(4) intermediate species F. ramosissima were determined using a pulse-chase technique with (14)CO(2)-(12)CO(2). The intermediate species F. ramosissima incorporated at least 42% of the total soluble (14)C fixed into malate and aspartate after 10 seconds of photosynthesis in (14)CO(2), as compared with 90% for the C(4) species F. trinervia and 5% for the C(3) species F. cronquistii. In both F. ramosissima and F. trinervia, turnover of labeled malate and aspartate occurred during a chase period in (12)CO(2), although the rate of turnover was slower in the intermediate species. Relative to F. cronquistii, F. ramosissima showed a reduced incorporation of radioactivity into serine and glycine during the pulse period. These results indicate that a functional C(4) pathway of photosynthesis is operating in F. ramosissima which can account for its reduced level of photorespiration, and that this species is a true biochemical intermediate between C(3) and C(4) plants.