The kinetics of synthesis and degradation of the protein constituents of nuclear membranes, endoplasmic reticulum membranes (rough-surfaced microsomes), Golgi apparatus membranes and plasma membranes were determined following a single administration of L- [guanido- 14 C] arginine by intraperitoneal injection. Membrane protein was determined as the fraction which resists sonication and sequential extrations with 1.5 M KCl, 0.1% deoxycholate and water to remove intravesicular, intracisternal (secretory), nucleo-, adsorbed and ribosome-associated proteins. The order of maximum labeling of membrane proteins was a) endoplasmic reticulum (nuclear membrane), b) Golgi apparatus, and c) plasma membrane. Rapid decreases in specific radioactivity followed maximal labeling of endoplasmic reticulum and Golgi apparatus membranes. These rapid turnover components of endoplasmic reticulum and Golgi apparatus were sufficient to account for labeling of plasma membranes via a flow mechanism. Incorporation of radioactivity into plasma membranes showed two distinct phases. The ultrastructural features underlying the biphasic pattern of incorporation into plasma membranes are discussed. Following initial incorporation and rapid turnover, membrane proteins were characterized by degradation kinetics approximating 1st order. Rates of degradation for Golgi apparatus and plasma membranes were faster than those for nuclear envelope and endoplasmic reticulum membranes. Assuming steady state conditions, an absolute synthetic rate of 7.1 mpg/min/avergage hepatocyte was calculated for membrane proteins of the plasma membrane. The results are compatible with intracellular movement and conversion of rough endoplasmic reticulum to plasma membrane via the membranes of the Golgi apparatus, i. e., membrane flow. Additionally, the kinetics indicate that membrane synthesis and transfer is restricted to specific parts of the endoplasmic reticulum and Golgi apparatus.