The synthesis, intracellular transport, storing, and excretion of proteins by duck hypophyseal cells in organ culture were studied with tritiated DL-leucine and high resolution radioautography (pulse-labeling experiments). Quantitative study of the radioautographs allowed a determination of the relative proportions of cytoplasmic radioactivity located in each cellular compartment (ergastoplasm, Golgi apparatus, and protein granules) as well as the variations in these proportions as a function of time. The number of labeled protein granules as opposed to the total number of granules in the cell was also determined (RSg). These data were separately analyzed for the two types of cells present in the explants: prolactin cells and "MSH" cells. The synthetic process follows a course common to both cell types, each of which is distinguished by its particular modalities. The labeled proteins, synthesized within several minutes in the ergastoplasm, are concentrated in the Golgi zone within 30 min. They then migrate out of this area, the emptying of which is accomplished in about 4 hr. These proteins become equally distributed between the protein granules, on the one hand, and the cytoplasm ("sedentary" proteins), on the other. The RSg reaches its maximum when the Golgi zone is emptied, but this figure remains very low (3%). The RSg then decreases slowly (1% in 40 hr). It is concluded that hypophyseal cells are able to store protein in their granules and that their processes of synthesis and excretion are not continuous. The prolactin cells differ from the "MSH" cells in that they have a slower migration of newly synthesized proteins, and these proteins pass via the dilated ergastoplasmic cisterns in which they may possibly be stored.