We describe a quantitative analysis of the internal structure of cardiac plasma membranes freeze fractured in situ, including the P-face particle density (lambda), the P-face particle diameter (d), the percent of fracture face area occupied by particles (Ap), and the spatial distribution of particles (random, clustered, or ordered). This analysis has been applied to seven sheep hearts to compare the plasmalemmal internal structures in ventricular and atrial myocytes and Purkinje strands of the same hearts and also to myocytes of frog, chicken, rabbit, and rat ventricles (to compare internal plasmalemmal structure of different vertebrate classes). Measurements were made on tissues conventionally prepared for freeze fracture by glutaraldehyde fixation and cryoprotection. We found that, in the same sheep hearts, lambda and Ap for ventricular plasmalemma significantly exceeded those for atrial plasmalemma and that the distribution of atrial P-face particles was more clustered than that for ventricle. d and Ap for frog ventricular plasmalemmal P-face significantly exceeded values for some of the other vertebrates.