The CO(2)/O(2) specificity factor of sucrose gradient purified ribulose 1,5-bisphosphate carboxylase/oxygenase from the C(3)-C(4) intermediate plants Moricandia arvensis (79 +/- 1) and Panicum milioides (89 +/- 2) was similar to the respective values of the enzyme from the closely related C(3) species, Moricandia foetida (80 +/- 5) and Panicum laxum (86 +/- 2). Thus, the kinetic properties of this bifunctional enzyme do not explain the reduced rates of photorespiration exhibited by either of these intermediate species.Dark/light ratios for aminoacetonitrile-sensitive (14)CO(2) evolution during decarboxylation of exogenous [1-(14)C]glycine by leaf discs had values of 9.0 with M. arvensis and 11.8 with P. milioides. Equivalent ratios with M. foetida and P. laxum were 2.5 and 3.2, respectively. Similar results were obtained using [1-(14)C]glycolate as the exogenous photorespiratory substrate, with dark/light (14)CO(2) evolution ratios for the C(3)-C(4) and C(3) leaf discs averaging 6.6 and 2.0, respectively. Stimulating photosynthetic CO(2) fixation by progressively increasing photon flux density from 0 to 1900 micromoles per square meter per second caused a concomitant reduction in (14)CO(2) evolution from leaf discs of M. arvensis and P. milioides supplied with [1-(14)C]glycine. Conversely, inhibition of photosynthesis by DCMU or the Calvin cycle inhibitor dl-glyceraldehyde increased (14)CO(2) evolution in the light to rates comparable to those in the dark. The data suggest that P. milioides and M. arvensis are capable of a more efficient internal recycling of photorespiratory CO(2) via ribulose bisphosphate carboxylase/oxygenase than closely related C(3) plants, and that this may partially account for the reduced rates of apparent photorespiration by these intermediate species.