A three-dimensional study of the sarcoplasmic reticulum (SR) of Purkinje cells of the goat heart was carried out with the use of serial thin sectioning. Two interconnected forms of SR were distinguished: myofibrillar SR and junctional SR (JSR). An extensive network of tubules lying outside as well as inside the myofibrils, constitutes the myofibrilar SR. The JSR, consists of vesicles, tubules and flattened cisternae. A three-dimensional model of a JSR cisterna was built showing its connection with both the myofibrillar SR and the sarcolemma where it forms a peripheral coupling. The potentiality and limitations of the serial-section technique for the study of the SR are discussed in comparison with other recently developed methods.