A study of the metabolic pathways of H(3)-thymidine utilization in buds of Lilium longiflorum and root tips of Vicia faba was undertaken in order to obtain information that might explain the binding of H(3) from H(3)-thymidine in the cytoplasm of these plants. H(3)-thymidine was administered for various periods of time, the tissues were fixed and processed in the manner routinely used in preparation for sectioning and autoradiography, and the radioactivity removed in this way from the tissues was determined. It was found that the ethanol/acetic acid fixative contained the major portion of the radioactivity. Analysis of this extract by paper chromatography showed that the radioactivity was distributed among various degradation products of thymidine, principally beta-ureidoisobutyric acid and beta-aminoisobutyric acid. Time course experiments with Vicia showed that these degradation products rapidly appeared in the tissue during incubation with H(3)-thymidine, while H(3)-thymine appeared in the incubation medium. Preliminary studies indicated that Vicia root tips incubated with H(3)-dihydrothymine for 24 hours would bind a small amount of H(3) non-specifically in the cells. It seems unlikely that utilization of degradation products of H(3)-thymidine is sufficient to explain labeling which is concentrated in the cytoplasm.