We have localised the placental endothelial marker caveolin-1 at the ultrastructural level using indirect immunogold labelling. The particulate label has been quantified to assess the distribution of the target protein within term placental chorionic villi. The mesodermal compartment of the tissue was more heavily labelled than the ectodermally derived trophoblast. Basal plate lining endothelium, and villous endothelium had similar immunoreactivity with anti-caveolin-1 antibody. A polarised distribution of the caveolin within chorionic villous capillary endothelial cells was observed. As evidenced by immuno-reactivity, the protein was statistically significantly more concentrated in the region associated with the basal membrane than the apical membrane. The latter region contained in turn significantly more anticaveolin-1 immunoreactivity than the central region. These differences are discussed in the light of possible transport and signalling platform roles for villous and basal plate endothelium. (c) 2006 Elsevier GmbH. All rights reserved.