In the hamster, guinea pig, rabbit, dog and cat, the right and left atria and ventricles were examined by immunohistochemistry, and the right auricular cardiocytes were studied by transmission electron microscopy. Moreover, ANP-granules in the cardiocytes were analyzed by ultrastructural morphometry. Immunohistochemically, the most intensely ANP-reactive cardiocytes were localized in the right auricle, particularly more prominent in the hamster and guinea pig than in the rabbit, dog and cat. The immunoreaction in the dog and cat was weaker than that in the rabbit. ANP-immunoreactivity was not detected in the ventricular myocardium of any of all species examined, but was occasionally observed in the subendocardium of the ventricular septum. Ultrastructurally, ANP-granules were localized principally in the perinuclear region associated with the Golgi apparatus and scattered throughout the sarcoplasmic layers. The Golgi apparatus of the cardiocytes was better developed in the hamster and guinea pig than in the rabbit, dog and cat. It was poorly-developed in the dog and cat. By ultrastructural morphometry, the number of granules was greatest in the hamster followed by the guinea pig, rabbit and dog or cat, in this order. On the other hand, the diameter of granules was largest in the guinea pig and reduced via the hamster to the rabbit. The diameter was significantly smaller in the dog than in the rabbit. The diameter of granules of the cat was lay between the rabbit and dog.