Brains of small, adult laboratory animals, fixed by formalin-perfusion, and cut into pieces usually 2-3 mm thick X 3-5 mm 2 are placed into a solution containing: 7% aqueous solution of potassium dichromate 100 ml,99.5% ethyl alcohol 40 ml, concentrated formalin 10 ml, distilled water 50 ml, for 24 hours. The process is repeated with a freshly prepared solution for another 24 hours. The pieces are then transfered to a 3.5% aqueous solution of potassium dichromate for 48hours, and then to a 1% aqueous solution of silver nitrate for 48 hours. Frozen or celloidin sections 70-100μm in thickness are put into 90% and then into 100% ethyl alcohol for 30 minutes respectively, creosote-benzene (1: 1), benzene and mounted with or without cover glass applying Entellan neu. This procedure gives high quality impregnation and it takes about one week to complete.