Photosynthesis and concurrent C export rates in source leaves of 14 ' Panicum' and 7 'Flaveria' C3, C3-C 4 intermediate and C4 species were measured using steady-state 14CO2 labelling at ambient (35 Pa) and high (90 Pa) CO 2. At ambient CO2, photosynthesis was strongly correlated with concurrent export for both the 'Panicum' (r = 0.97) and ' Flaveria' (r = 0.76) genera. Within the 'Panicum' genus, photosynthesis and export were highest in C4 species, lower in C3-C4 intermediates and lowest in the C3 species. However, when export was expressed as a percent of photosynthesis, "type I" C3-C4 intermediates had export rates similar to C4 species. The C4 NADP-ME and PEP-CK species had the highest export rates among the C4 subtypes. Among ' Flaveria' species, export was highest in C4 species, but the "type II" C3-C4 intermediates and not the C3 species had the lowest immediate export rates. At high CO2, export in the C3 and C3-C 4 intermediates of both genera did not increase proportionally with photosynthesis. Export was lower in some C4 species. In all ' Panicum' and 'Flaveria' species export, as a percent of photosynthesis, decreased at high CO2. All species accumulated starch and sugars at both CO2 levels. At high CO2, the C4 'Panicum' species accumulated only additional sugars, whereas the C4 'Flaveria' species accumulated only additional starch. Overall, however, there was no discernable labelling of sugars and starch (i.e., storage products) which correlated with immediate export rates among C3, C3-C4 intermediate and C4 photosynthetic types within each genus. Diurnal export patterns as determined by pulse-chase experiments at ambient CO2, indicated that the 'Flaveria' species which had low immediate export did not compensate during subsequent chase periods in light and dark. 'Panicum' C3-C4 intermediate and some C4 species appeared to compensate by exporting more during the chase periods, but not the C3 species. Concurrent export from leaves of the 'Flaveria' genus correlated well with whole-plant growth parameters suggesting that the immediate export rate during photosynthesis may be used as a predictor of whole-plant growth. These correlations were stronger than those obtained using leaf photosynthesis. However, poor correlations were obtained among all leaf and whole-plant parameters of the 'Panicum' species.