The present report describes a procedure developed for tracing C¹⁴ labeled glucose in rumen material in vitro. Twenty-five ml. samples of rumen fluid were incubated with uniformly labeled glucose for 1 1/2 hours in a closed system. The distribution of activity in both unfermented control samples and fermented samples was determined among protein and particulate material, ether extractable substances, amino acids and peptides, sugars and other neutral molecules, carbon dioxide, and methane. Samples were counted as barium carbonate following wet oxidation. Subsamples of the control and fermented samples indicated 80 to 85% recovery of C¹⁴. In these preliminary trials, additional losses were encountered among some individual fractions. Data indicated differences between fermentative activity of the two samples from different animals. In Trial I, 29% of 14 the fermented glucose C¹⁴ was recovered in the protein-particulate fraction, 15.6% in the carbon dioxide, and 10.5% in the ether extract. In Trial II, the ether extractable fraction contained 23.5% of the activity, 14.5% was recovered in the protein-particulate fraction, and 14.3% in the carbon dioxide. / M.S.