Phase contrast microscopy of cultured embryonic heart cells showed the beating frequency decreased more rapidly and the regularity the rhythm of of the beating cells was lost sooner in heart cells from cardiomyopathic hamsters than from the control hamsters. Studies of cultured heart cells by differential interference contrast (with Nomarski's prism) and by electron microscopy revealed a significant impediment in the maturation of the sarcomeric units in the diseased animals compared to controls. The incorporation of [14C] leucine into acid-insoluble fractions was studied, and no significant difference in incorporation between the two groups was found. An analysis of polyacrylamide gel electrophoresis revealed the possible existence of a quantitative difference in one of the composing proteins of the erythrocyte membrane between the two groups. The protein kinase activity of ghosts from the control group was more sensitive to cAMP than that from the diseased animals. In addition, the binding of [3H] cAMP to the ghost was almost identical between the two. The morphological and biochemical observations lead one to the plausible supposition that there are some differences in the interaction of the so-called catalytic and regulatory subunits between the two groups and that there is an impairment of the higher arrangement of myofibrils from their building blocks in the diseased hamster. The significance of the existence of abundant corpuscles resembling neurosecretory granules was not established by this study. They may have an etiological significance or they may be related to a disturbed function in the cultured cells of the cardiomyopathic hamster.