Rat embryo fibroblasts, grown in Eagle's MEM with 10% serum, showed a rapid increase in autophagic vacuoles when placed in MEM with 0–1% serum. Concurrent with this response, degradation of cellular proteins showed a 2‐fold increase. We did not find any increases in cathepsin D, β‐glucuronidase, β‐galactosidase, and β‐glucosidase, or proteolytic activity of cell homogenates at pH 3.7 towards endogenous substrates. Homogenates prepared in 250 mM sucrose at pH 7.0 showed a 40% increase in protein breakdown. These data support the hypothesis that the induced increase in proteolysis, characteristic of cells placed in a nutritionally deficient medium, is effected by an activated vacuolar apparatus (lysosomes and autophagic vacuoles). We suggest, however, that this mechanism is distinct from normal protein turnover in the cell, but can be rapidly induced by appropriate alterations in the cellular environment. Finally, this induced proteolytic mechanism is not dependent upon an increase in lysosomal enzymes, but rather a structural alteration within the cell which effects a transfer of cellular proteins into the vacuolar apparatus.