Localization of serum albumin in the striated and smooth muscles of rat was studied by an improved immunocytochemical method. Diaphragm, ventricular myocardium, and smooth muscle of stomach were examined. In all of these tissues, albumin was found in the interstitial space and small subsarcolemmal caveolae and vesicles. In addition, the transverse tubular system of the striated muscle stained positive for albumin. The subsarcolemmal vesicles containing albumin did not show any evidence of fusion with lysosomes. Furthermore, in smooth muscle, most of these vesicles were open to the extracellular space. These results demonstrate that albumin in smooth and striated muscle is confined to the extracellular space suggesting that substances such as fatty acids which are carried by albumin are split from it and taken up at the level of the plasma membrane.