A study was conducted on a C(4) (Panicum maximum) and a C(3) (Panicum bisulcatum) species to determine the nature of the dark release of (14)CO(2) with respect to its responses to changes in temperature and O(2) tension during light CO(2) uptake of (14)CO(2).The total one minute uptake of (14)CO(2) increased with increased temperatures up to 30 degrees C for both species at all O(2) tensions.The total release of (14)CO(2) during the first four minutes of dark by the C(4) species was 5 to 10 times greater than the C(3) species; however, there was no consistent pattern in the C(4) species as a function of temperature or O(2) tension. In contrast, the dark release by the C(3) species was similar to that of light uptake as a function of temperature or O(2) tension. The percentage of the one-minute (14)CO(2) fixation that was released as (14)CO(2) during the first four minutes of dark increased with O(2) tension for the three highest temperatures (20, 30, and 38 degrees C) and remained, though reduced, after a 10-minute dark chase in (12)CO(2) for the C(3) species.IT WAS CONCLUDED THAT: (a) the source of the dark CO(2) release in C(3) species is different from that of C(4) species; (b) the dark CO(2) release in C(3) species likely is derived from intermediates of photorespiration and is similar to the postillumination burst of CO(2); and (c) the source(s) of the dark CO(2) release in C(4) species is not photorespiration.