Section I. A Method For Determination of Carbon Dioxide Specific Radioactivity and Concentration in Blood, Rumen Fluid, and Expired Air. A method is described for determining carbon dioxide specific radioactivity and carbon dioxide concentration in blood, rumen fluid, and expired air. The carbon dioxide of blood and rumen fluid samples is selectivity liberated by 1 M phosphate buffer (pH 5.8) containing 1 mM NaN(,3) and 5 mM NaF. Liberated carbon dioxide is quantitatively trapped in Ba(OH)(,2) and amounts of carbon dioxide determined by titration of unconsumed hydroxide ion. The trapped BaCO(,3) is solubilized with 10% EDTA in 1 M THAM (pH 9) and radioactivity determined by liquid scintillation. For quantitative determination of expired carbon dioxide, a measured fraction of expired air is bubbled through Ba(OH)(,2). The amount of carbon dioxide trapped is determined by titrating an aliquot of the trapping solution after centrifugation to precipitate the BaCO(,3). The carbon dioxide is regenerated by addition of acid and quantitatively retrapped in ethanolamine. Radioactivity is determined by liquid scintillation of ethanolamine carbamate;Section II. Recycling of Carbon in Ruminant Gluconeogenesis. Various components of recycling of glucose carbon were measured in three Holstein steers weighing from 200 to 276 kg and fed 385 g of a 70:30 roughage:concentrate diet every two hours. Total recycling of glucose carbon ranged from 50 to 64 mmol carbon min('-1). Recycling via carbon dioxide, measured directly, was 0.4 mmol carbon min('-1) and was 1 mmol carbon min('-1) via pyruvate-lactate, measured by combination of 6-('3)H- and U-('14)C-glucose tracers. Total chemical recycling, assessed by combination of 2-deoxy-glucose and U-('14)C-glucose, was 17 to 22 mmol carbon min('-1) and physical recycling was 40 to 47 mmol carbon min('-1). Components of chemical recycling other than crbon dioxide and lactate and components of physical recycling could not be defined. Limited data from steers on other dietary regimes are presented.