Newly formed neurons in the adult mammalian neocortex have been reported by several investigators using light microscopic radioautography, but these reports have not been confirmed by electron microscopy--probably because their rarity precludes any reasonable chance of observing these cells with electron microscopic radioautography. To overcome this problem I have used a recently developed method that allows serial thin sectioning and subsequent electron microscopic examination of plastic-embedded sections previously prepared for light microscopic radioautography. Ninety-day-old rats were injected with 4.3 microCi per gm body weight of [H3] thymidine and allowed to survive for 30 days. In the light radioautographs, labeled cells were found in layer IV of the visual cortex, and analysis of electron micrographs of selected examples of these labeled cells clearly demonstrated their neuronal nature wit synapses along their cell bodies and dendrites. In order to quantify the relative frequency of labeled neurons, the number of labeled cells seen in the light microscopic sections was expressed as a percentage of the total number of neurons found in sections through the entire thickness of the visual cortex; the percentage was 0.011%, or about 1 in 10,000. The results of this study are in agreement with evidence of neurogenesis of granular neurons in the adult rat olfactory bulb and dentate gyrus (Kaplan and Hinds, '77). Thus, it has now been confirmed that relatively small labeled neurons and their synapses are found in at least 3 brain regions (olfactory bulb, dentate gyrus, and visual cortex) in a normal adult rodent.