Glucose utilization by the marmoset salivary glands was studied radioisotopically, both in vitro and in vivo. Surgically excised, minced parotid and submandibular glands from 7 adult animals were incubated in vitro in a medium containing uniformly labeled 14C glucose (glucose-UL-14C), then subjected to respirometry measurements and to extraction and separation procedures for the radioautographic isolation and identification of amino acids, organic acids, and glucose. Glucose utilization in vivo by the marmoset salivary glands was determined in 3 animals by analysis for 14C-containing amino acids, organic acids and glucose in pilocarpine-stimulated whole saliva, following intraperitoneal injection of glucose-UL-14C. The whole saliva was fractionated, separated and processed for localization of radioactivity by the same methods that were used in the in vitro studies. The isolated and in situ salivary glands converted glucose-UL-14C into 14C-containing carbon dioxide, glycine, alanine, glutamic acid, aspartic acid, and lactate. In no instance did the amount of 14C-glucose secreted in saliva exceed 0.52 mg/100 ml..