A combined technique of formaldehyde-induced fluorescence (FIF) and autoradiography is described for the localization of radioactively labeled substances in relation to monoamine neurons. This method permits the simultaneous visualization of 3H-labeled steroid hormone or drug uptake sites and fluorescing monoamine neural elements (cell bodies, fiber projections, terminals) in the same tissue section. Thin frozen sections cut in a cryostat are freeze-dried, exposed to formaldehyde vapor at 80°C, and carried through dry-mount autoradiography processing steps before fluorescence microscopy screening. Subsequent histological staining of sections and light microscopy are employed for conventional autoradiogram screening. With this procedure, 3H-estradiol and 3H-dihydro-testosterone are localized in various catecholamine (CA) neurons in the diencephalon and lower brain stem of the rat. Also, catecholaminergic as well as noncatechol-aminergic sex steroid target neurons are seen to be innervated by CA terminals in various rat brain regions.