P-face particle density of freeze-fractured cardiac plasmalemma provides an estimate of the total population of membrane channels, carriers, and receptors that insert into or traverse unit area of lipid bilayer. We have determined the size of this population of integral membrane protein assemblies by counting the number of P-face particles per square micron of plasmalemma in freeze-fractured hearts, using stereoimaged replicas tilted with a goniometer stage in the electron microscope. Particle numbers per square micron were 4,525 +/- 231, 4,799 +/- 235, 4,122 +/- 153, 4,281 +/- 218, and 5,848 +/- 300 for ventricular myocyte plasmalemmas of rat, rabbit, 8-day chick, adult chicken, and frog, respectively. These values are at least two times greater than published values, in which stereo imaging was not used. Published ligand-binding studies indicate that only the surface density of the Na+-K+ pump sites can account for a significant fraction of P-face particles; the rest so far lack functional correlates. Particle density of frog heart plasmalemma significantly exceeded particle densities of chicken and mammalian plasma membranes.