It is known that colonic goblet cells utilize glucose to synthesize the carbohydrate portion of mucus glycoprotein. To determine the intracellular site of this synthesis, glucose-H(3) was injected into 10-g rats. At 5, 20, 40 min, 1, 1(1/2), and 4 hr after injection, segments of colon were fixed and prepared for electron microscope radioautography. By 5 min after injection, label had been incorporated into substances present in the flattened saccules of the Golgi complex. At 20 min, both Golgi saccules and nearby mucigen granules were labeled. By 40 min, mucigen granules carried almost all detectable radioactivity. Between 1 and 4 hr, these labeled granules migrated from the supranuclear region to the apical membrane; here, they were extruded singly, retaining their limiting membrane. The evidence indicates that the Golgi saccule is the site where complex carbohydrate is synthesized and is added to immigrant protein to form the complete glycoprotein of mucus. The Golgi saccule, distended by this material, becomes mucigen granules. It is roughly estimated that one saccule is released by each Golgi stack every 2 to 4 min: a conclusion implying continuous renewal of Golgi stacks. It appears that the Golgi synthesis, intracellular migration, and release of mucus glycoprotein occur continually throughout the life of the goblet cell.