The metabolic products of inorganic carbon taken up through the roots from nutrient solution were studied in willow plants. Willow cuttings (Salix cv. Aquatica gigantea) were supplied with unlabelled or 14C-labelled NaHC03 for 1, 5, 10, and 24 h in light or in darkness. After feeding, the plants were divided into six samples (upper and lower leaves and corresponding stems, cuttings and roots), which were frozen in liquid N2. Freeze-dried ground samples were extracted into water-soluble, chloroform-soluble and insoluble fractions. The water-soluble fraction was further separated into basic, acidic, and neutral fractions by ion-exchange chromatography. In the light experiment pronase treatment was used to separate the insoluble fraction into proteins and insoluble carbohydrates. After I h feeding time, most of the 14C was fixed into organic acids and amino acids both in light and in darkness in all parts of the plants. In the roots a large part of the l4C-carbon was incorporated into the protein and insoluble fractions already during short feeding times, and the amounts incorporated increased with time. In the leaves, after 1 and 5 h the main labelled compounds were the organic acids and amino acids, but after 10 h about half of the total 14C was in protein and in the insoluble fraction. A further analysis of amino acids and organic acids with HPLC showed that C-4 acids were labelled initially and that over time the proportion of different acids changed. These results indicate that the metabolism of carbon in roots might take place via β-carboxylation of PEP. Part of the fixed 14C is transported from the roots, probably as amino acids and organic acids, to the shoot. In roots the C-4 acids are metabolized further into structural compounds (proteins and insoluble carbohydrates).