Spontaneous mineralization observed in the thalamus, and meninges of the median longitudinal fissure, of several strains of mice was studied by light and electron microscopy. The mineralized deposits were closely associated with blood vessels, were periodic acid-Schiff positive, and contained variable amounts of calcium. The thalamic deposits were found to be in an extra-cellular location, and consisted of a randomly arranged mass of fine fibrils or spicules, with laminations of varying electron density in larger deposits. The deposits appeared to develop on the surface of subendothelial and medial, vascular basement membranes. Energy dispersive, X-ray microanalysis demonstrated the presence of both calcium and phosphorus in small and large deposits.