The dependence of cytoplasmic membranes upon the nucleus was studied by examining enucleated amebae with the electron microscope at intervals up to 1 wk after enucleation. Amebae were cut into two approximately equal parts, and the fine structure of the enucleated portions was compared with that of the nucleated parts and starved whole cells which had been maintained under the same conditions. Golgi bodies were diminished in size 1 day after enucleation and were not detected in cells enucleated for more than 2 days. The endoplasmic reticulum of enucleated cells appeared to increase in amount and underwent changes in its morphology. The sparsely scattered short tubules of granular endoplasmic reticulum present in unmanipulated amebae from stock cultures were replaced in 1-3-day enucleates by long narrow cisternae. In 3-7-day enucleates, similar cisternae of granular endoplasmic reticulum encircled areas of cytoplasm partially or completely. It was estimated that in most cases hundreds of these areas encircled by two rough membranes were formed per enucleated cell. The number of ribosomes studding the surface of the endoplasmic reticulum decreased progressively with time after enucleation. In contrast, the membranes of nucleated parts and starved whole cells did not undergo these changes. The possible identification of membrane-encircled areas as cytolysomes and their mode of formation are considered. Implications of the observations regarding nuclear regulation of the form of the Golgi apparatus and the endoplasmic reticulum are discussed.