Using microinjection of probes detectable by epifluorescence light microscopy and electron microscopy we have provided direct evidence that syncytiotrophoblast is a true syncytium. We have investigated the freedom of movement of probes injected into the syncytiotrophoblast. These probes appear to move relatively extensively within the cytosolic compartment which forms a ‘maze’ through the membrane-bounded organelles and reticula of the syncytium. Thus mathematical models of certain forms of transepithelial transport based on simple mean and harmonic mean histologically measured thickness of epithelia are unlikely to describe the transepithelial transport pathway of at least some larger molecules, particulates and organelles with great accuracy. Microinjection of probes directly into the mesenchymal core of chorionic villi has been used to demonstrate extensive freedom of movement in that compartment which is the region where exchange with elements contained in the fetal vascular tree takes place.