We investigated the distribution of hyaluronan (hyaluronic acid) in rat vascular tissue fixed by an osmium tetroxide (or glutaraldehyde) microwave technique and embedded in epoxy resin (or Lowicryl K4M), using hyaluronan binding proteins coupled to 15-20-nm gold particles as ultrastructural markers in a one-step post-embedding procedure. The intra- and extracellular aspects of vascular endothelial and smooth muscle cell plasma membranes revealed distinct labeling, with a high affinity for caveolae being manifested in both cell types and in all kinds of vessels. Hyaluronan was also localized intercellularly in areas characterized by extensive endothelial cell interdigitation. Intracellularly, moderate staining of nuclear heterochromatin was observed.