Modifications of the single-section Golgi-impregnation procedure of Gabbott and Somogyi are described. The modifications allow easier and more rapid preparation of the sections for Golgi-impregnation and easier handling of large numbers of serial sections. The technique consists of placing a section that has been treated with osmium tetroxide and potassium dichromate on a microscope slide and "sandwiching" it with a second microscope slide. The two slides are held together at one end by tape and the assembly is dipped into a solution of silver nitrate. Golgi-impregnation of neurons occurs within a few hours and is generally complete within 12 h. The technique has been applied to sections through the caudate nucleus of the cat and ferret in order to define the morphological characteristics of striatal substance P- and methionine enkephalin-immunoreactive neurons. Sections were first incubated to reveal the immunoreactive structures and then subjected to the Golgi method. Golgi-impregnated neurons that were immunoreactive for either substance P or methionine enkephalin had medium-size perikarya from which several dendrites emerged. The dendrites branched close to the perikaryon; secondary and higher order dendrites were densely laden with spines, as many as 15 spines per 10 microns of dendrite. It is concluded that both striatal substance P-containing and methionine enkephalin-containing neurons are of the medium-size densely spiny type. Medium-size densely spiny neurons may be homogeneous with respect to their somatodendritic morphology but heterogeneous with respect to their chemical characteristics and axonal morphology.