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A time-saving method is described for the localisation of chronically implanted electrodes in the brain of test animals by means of a "Kryostat" hard section microtome which allows both histo-topographic and histo-enzymatic investigations. This is made possible by the incorporation of a three-dimensional microtome holder.

A fast and simple method is described for the localisation of tips of chronically implanted electrodes in the brain of rabbits which may be obtained--under standard conditions--by an intra-aortal injection of a 2% potassium-hexacyanoferrate-III solution. This method also facilitates histological preparations with a thickness of 5 mu only and thus permits a thorough morpho-pathological examination not only in the area of the electrode tips but in adjacent zones as well. The use of a buffered formalin solution also permits certain histochemical examinations to be made, for instance the determination of nucleic acids.

A modified stereotaxic surgical and implantation technique for animal EEG studies in rabbits is described. The use of a stereotaxic "socklet unit" with a minimum of operational equipment, high accuracy, simple handling, and good tissue compatibility permits the repetition of EEG studies in the same animal with a large number of electrodes and over a period of several months. For example for this technique an experimental arrangement for recording visually evoked potentials is described. The importance of a careful selection of the experimental animals, of maintaining constant test conditions, and of the selecting of test parameters as a prerequisite for assessing induced alterations is emphasized and discussed.