SUMMARYTechniques for the preparation and investigation of thick biological sections in the high voltage electron microscope (HVEM) have been studied using the Toulouse 1.5 and 3 MV microscopes.Sections several micrometres thick can be made using conventional ultra‐microtomes, but, in order to know their actual thickness, it is necessary to resection them perpendicularly to their surface after embedding them in Araldite.When successive thick sections are made with glass knives, damage may be observed on their upper surface. This can be avoided by alternating several thin sections between each thick section. Such damage is further reduced if thick sections are made using diamond knives.The maximum thickness that can be observed at a voltage of 2.5 MV is not limited by the penetrating power of the beam but by the damage suffered by irradiated Araldite. The limit is approximately 10 μm. Below 10 μm a general uniform loss in thickness is observed in irradiated areas; above 10 μm, that loss in thickness is irregular and craters are formed under the beam.Contrast can be obtained by staining tissues before or after embedding them in Araldite. The cellular structures can be uniformly stained using uranium or lead salts, or selected parts of them can be stained using colloidal tracers, Gomori's lead method, osmium or silver impregnations. The latter techniques are the more convenient as far as sections several micrometres thick are concerned.The resolving power depends on section thickness and accelerating voltage. At 2.8 MV, by using selective staining techniques, sections several micrometres thick can be investigated with a sufficient resolving power, especially with regard to understanding the spatial organization.Stereoscopic viewing allows a better interpretation by removing the inconveniences resulting from overlapping structures, but the depth of field must then be considered. Its value, for a given resolving power, taking into account the focal length of the objective lens, depends on the objective aperture: by using a 5 μm aperture one can get a depth of field of 4 μm at a resolving power of 2.0 nm and 12 μm at a resolving power of 6.0 nm, the focal length of the Toulouse 3 MV electron microscope being 10 mm. At this voltage aberrations due to diffracting phenomena are not very dis