Rat DRG cells at early stages of development were observed to clarify the forming process from bipolar to pseudounipolar cells. Dissociated DRG cells in vivo and in vitro were observed by light and scanning electron microscopy. The percentage of cell types in the 13th prenatal to the 1-day postnatal DRG in vivo were investigated. It is noteworthy that about 5% of ganglion cells were still bipolar in the 1-day postnatal rat. When dissociated DRG cells were seen in long-term culture, completely unipolarized DRG cells were revealed by silver-impregnation. But when fibroblast growth was suppressed in long-term culture, no pseudounipolar cells were observed. Dorsal root ganglia on day 14 of gestation were organ cultured. The incidence of pseudounipolar cell appearance was estimated time-sequentially by silver-impregnations (Table 2). Unipolar cells increased to about 80% at the 22nd day of culture. Cultured DRG cells were examined transmission electron microscopy and the morphological change from bipolar cells to pseudounipolar cells was investigated. No evidence was obtained which showed that the processes of bipolar cells fuse to make a stem process. In conclusion, observations of developing DRG cells by light and electron microscopy revealed that the stem process of the pseudounipolar DRG cells is derived from the elongated cytoplasm of the cell body and is not derived from the fusion of the opposite two processes.