In a previous study, we examined the kinetics of radioactivity evolution from rhizosphere respiration after the pulse labelling of maize shoots with (CO2)-C-14 (Nguyen et al., 1999). The specific activity of rhizosphere respiration demonstrated two peaks of (CO2)-C-14 production. The first one occurred a few hours after the pulse of (CO2)-C-14 and was followed by a second peak, which took place during the night following the labelling. In the present work, we demonstrate that the second phase of activity occurred in both sterile and non sterile plant-soil systems. This was inconsistent with the results obtained for wheat by Warembourg and Billes (1979) who observed the second peak solely in the case of non-sterile cultures. These authors suggested that this second phase of (CO2)-C-14 production was related to microbial mineralisation of labelled complex compounds. Their synthesis and breakdown into smaller molecules delayed their utilisation by micro-organisms. However, in the present work, we also demonstrate that the second phase of activity was closely related to photoperiod. When plants were transferred from a 16 h to 20 h photoperiod, the second mineralisation of labelled rhizosphere compounds occurred sooner after the initiation of the dark period and it was strongly attenuated. Therefore, we suggest that the second phase of activity resulted from the utilisation by roots and by micro-organisms of stored C-14-compounds, which accumulated during the previous light period.