A statistical method for reconstruction of the distribution of the diameters of spheres according to their planar image obtained from ultrathin sections, is proposed. The method can be applied for casses when the thickness of the section is commensurate with the diameter of the spheric objects studied. The formulae worked out in the work are based on the assumption that the diameters of the spheres are with discrete distribution in geometric order. The method is elaborated in connection with the problem of the changes in the diameters of the dense-core vesicles in monoaminergic structures during development.