Experiments in which glycine-3H has been introduced into excised neurointermediate lobes of Xenopus laevis incubated in a modified Krebs-Ringer bicarbonate medium have shown that ∼ 50% of the incorporated radioactivity is present in small peptides which have an electrophoretic mobility characteristic of the melanocyte-stimulating (MSH) peptides shown to be elaborated within the tissue. Based on these results and the demonstration that a discrete ∼ 7 min pulse of the label can be introduced into the tissue, electron microscope radioautography has been employed to follow the subcellular events concerned with the synthesis, intracellular transport, and packaging of the labeled secretory product. Together, these studies indicate that the newly synthesized material arises in peptide form, rather than as part of a larger prohormone molecule, on the ribosomes of the rough endoplasmic reticulum within the parenchymal cells of the intermediate portion of the lobe. A proportion is then incorporated into and remains for an extended period within the intracisternal granules which are a feature of the rough endoplasmic reticulum within these cells in vitro Most (∼ 60%) of the labeled secretory product, however, is transferred to the Golgi complex within 30 min and, within a further 10 min, becomes packaged into small (∼ 200 mµ) electron-opaque secretory granules. It is probable that under the conditions employed these granules represent the final intracellular location of secretory product before it is released

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