The buffered picric acid paraformaldehyde fixative originally recommended for electronmicroscopy and which has since been used occasionally for light-microscopic immunocytochemistry, has been supplemented with glutaraldehyde and used as primary fixative for the perfusion of rat brains. In the basal ganglia and preoptic area, substance P, somatostatin and leu-enkephalin immunoreactive material was localized with the unlabelled antibody enzyme method in thick sections cut from freeze-thaw treated blocks. Good penetration of the antibodies without the use of detergents and the light background of the osmium-treated sections allowed the selection for electron-microscopy of immunoreactive structures as small as individual boutons that had been identified at the light-microscopic level. It is suggested that the procedure may be useful for electron-microscopic sampling of immunoreactive structures occurring infrequently over a large area or for the electron-microscopic study of light-microscopically classified neurons.