We developed an efficient method of optical clarification of isolated structures in the brain of mature mice. The method decreases optical density and opens the way to use novel methods of optical tomography in the study of the brain tissue in mature organism. Specimens dehydrated with 2-butoxyethanol, destained with Dent solution, and cleared in benzyl benzoate:benzyl alcohol or in water solution of iothalamate had the lowest optical density. The clearing effect did not directly depend on the refraction index of the composition. Possible mechanisms of increasing optical transmission of myelinated structures in the brain are discussed.