This paper reports light and electron microscope radioautographic studies of the sites of gastrin synthesis and the paths of intracellular migration of secretory granules in G-cells of rat glandular stomach incubated in vitro with 3H-glutamic acid and/or 3H-glycine at pH 8.2 or 2.5. Although the ultrastructural preservation of tissues maintained in pH 2.5 medium deteriorated within 30 minutes after beginning incubation, morphological observation was possible at least 60 minutes in the pH 8.2 medium. At 5 minutes, silver grains were few, and located chiefly over granular endoplasmic reticulum. After 30 minutes of labeling, silver grains were more numerous and found over the cytoplasm rich in secretory granules and over the nucleus. After incubation for 60 minutes, the distribution of silver grains was the same as at 30 minutes incubation but the labeling was heavier. Secretory granules of different sizes and electron densities were not differentially labeled. The following conclusions are drawn: (1) glutamic acid and/or glycine are incorporated in G-cells, synthesized into proteins and/or polypeptides in the rough surfaced endoplasmic reticulum and formed into secretory granules probably in the Golgi area: (2) the secretory granules thus formed migrate from the Golgi zone to the peripheral cytoplasm and are stored there; and (3) the kinetics of secretion in G-cells are fairly speedy under certain conditions.