Ultrastructural analysis of specific heart granules in the left atrial myocardium of the rat shows that the so-called A and B types are not different populations but represent varying planes of section through a uniform set of organelles. Histograms of the apparent diameters of granules in individual animals establish that they are unimodal and tilt series of micrographs through selected granules show that the appearances of A and B profiles are interchangeable during the tilting manoeuvre. Moreover, serial sections reveal that equatorial sections through a granule yield the typical appearance of A profiles while peripheral sections through the same granule yield a B type appearance. Immunocytochemical studies were conducted on acrylic resin-embedded specimens using antiserum raised to ANP-28 and visualized by labelling with protein-A gold. A type profiles are more heavily decorated with gold particles than B profiles, a feature which could be partially explained if there is a higher concentration of peptide at the centre of the granule core. Some peripherally sectioned caps of granules which project B type profiles do not exhibit immunoreactivity because their orientation within the thickness of the section prevents access of the antiserum but, after resectioning at right angles to the original plane, their ANP content is revealed by a second phase of immunogold labelling.