Labelling methods of photo-assimilates with 13C or 14CO2 are widely used to study the C transfer from the plant to belowground compartments because they permit to trace root-released C in soils. The main constraints for such artificial labelling of plants are i) hermetic separation of roots from shoots that avoid CO2 fixation by the roots (PEPcase) or fixation/exchanges on carbonates ii) regulation of climatic and labelling parameters (steady-state ratio of 14C/12C or 13C/12C) of the atmosphere around shoots. Those prerequisites, crucial in studies on recently assimilated C allocation to rhizosphere, are not always taken into account in the recent papers. I present here a procedure preventing such inconvenient that could distort data, as a consequence of an alteration of photosynthesis and C partitioning in the plant and rhizosphere. Prior to labelling, the belowground compartment is hermetically separated from the labelling atmosphere with liquid silicon set between shoots and roots. The 14CO2 (or 13CO2) is generated by the addition of NaH14CO3 (or NaH13CO3) in 1M lactic acid. CO2 and 14C specific activity of the atmosphere in the labelling chamber are regulated with a β radiations scintillation detector coupled with an infra-red gas analyzer (IRGA), mass flow controller for 12CO2 and a peristaltic pump for 14CO2 from NaH14CO3. For 13C labelling, 12CO2 and 13CO2 can be regulated separately with a dedicated IRGA. The required 14C/12C or 13C/12C ratio is reached within 10 min. after the beginning of the labelling and remains constant, regulated, as 12CO2, according to the shoot demand. The belowground compartment is continuously flushed with decarbonated air at the input, and CO2 evolved from this compartment at the output is either measured with an IRGA or trapped for measurement of respired 12C, 14C or 13C. At the end of labelling, air in the chamber is forced to pass through a soda lime trap in order to decrease the 14C/12C or 13C/12C ratio in the chamber, while 12CO2 is continuously injected in the chamber to compensate for trapping and shoot uptake. As a part of the 14C (or 13C) respired by the shoots during the chase period can be re-fixed during the chase period, the 14C (or 13C) is continuously fixed in a soda lime trap. Thus, the CO2 concentration is steady during all the procedure (commonly 360± 10 vpm max.). CO2 concentration can be regulated up to 1000 vpm. Pulse labelling or continuous labelling is feasible.