The hypothesis that cytoplasmic protein in rat liver is internalized and degraded intralysosomally during basal or steady state turnover as well as in deprivation-accelerated states was tested by (a) determining the subcellular location of degradable protein in homogenates and (b) comparing the amounts of this protein pool with rates of long lived protein degradation in livers perfused under the above conditions. Greater than 95% of total proteolysis in homogenates, measured at 37 degrees C from free amino acid generation or the release of [14C]valine in previously labeled livers, was associated with particulate fractions, and it paralleled lysosomal markers closely during graded differential centrifugation after Triton WR-1339 loading. Because 14C-labeled cytosolic proteins were not degraded by intact isolated lysosomes at pH 6 or 7, the protein substrate must have been internalized by lysosomes prior to homogenization. Time courses of intralysosomal proteolysis from maximal deprivation down to and including basal were identical in shape, and each leveled off abruptly after 120-150 min. Although the velocities of proteolysis were reduced in vitro, estimates of the degradable protein pools were closely proportional to corresponding rates of long lived degradation in perfused livers. In absolute terms, they fit recent predictions of internalized cytoplasmic protein based on stereological measurements of lysosomal volumes.