The impact of inoculation with Paxillus involutus on the utilization of organic carbon compounds by birch roots was studied by feeding [14C]Glu or [14C]malate to the partners of the symbiosis, separately or in association, and by monitoring the subsequent distribution of 14C. Inoculation increased [14C]Glu and [14C]malate absorption capacities by up to eight and 17 times, respectively. Six- and 15-d-old mycorrhizal roots showed about four-fold higher [14C]Glu and [14C]malate absorption capacities compared with 60-d-old mycorrhizal roots, suggesting that the early stages of mycorrhiza formation induced higher requirements for C skeletons. Moreover, the results demonstrated that inoculation strongly modified the fate of [14C]Glu and [14C]malate. It was demonstrated that exogenously supplied Glu and malate might serve as C skeletons for amino acid synthesis in mycorrhizal birch roots and in the free-living fungus. Gln was the major 14C-sink in mycorrhizal roots and in the free-living P. involutus. In contrast, citrulline and insoluble compounds were the major 14C sinks in non-mycorrhizal roots, whatever the 14C source. It was concluded that mycorrhiza formation leads to a profound alteration of the metabolic fate of exogenously supplied C compounds. The ecological significance of amino acid and organic acid utilization by mycorrhizal plants is further discussed.