We studied N-myc RNA by in situ hybridization and S1 nuclease protection analysis in human fetal cerebrum, retina, lung, liver, and placenta during the second trimester. High levels of N-myc RNA were found in the early fetal cerebral germinal layer and the primordial cortex, with lower levels in the intermediate layer. After the twentieth week, N-myc expression declined in the attenuated germinal layer, remained high in the undifferentiated outer cortex, but declined in the differentiating inner cortex, which now expressed c-src. The primitive retina had high levels of N-myc RNA in the inner nuclear and ganglion cell layers between 12 and 21 weeks of fetal age. During this time, c-src RNA increased with fetal age in the ganglion cell layer. Lower levels of N-myc RNA were expressed in some cells of lung and placenta. Thus, appreciable N-myc RNA elevation is present in immature neural cells, disappears with differentiation, and may be unrelated to mitosis since high levels occur in the primordial cortex, which grows by accretion, and not by cell division.