Because of the large number of cell phenotypes in the nervous system, it has been difficult to characterize each as to specific electrophysiological properties. We have developed a technique that allows the identification of central and peripheral nervous system neurons following intracellular recording. We use electrodes that contain 2% biocytin to do current- and voltage-clamp recordings; the recorded neurons are revealed with streptavidin-fluorescein isothiocyanate labeling and identified through immunohistochemical staining for specific antigens. Presently, we report on the use of this technique to identify four cell types--dopamine, beta-endorphin, vasopressin and oxytocin--in the hypothalamus of the mammal. This technique should have widespread applicability for electrophysiologists.