The morphological changes of olfactory mucous membrane have been investigated with light microscope and T.E.M. within a period of 3 days to 4 weeks after unilateral olfactory nerve sectioning. No obvious morphological changes of supporting cells and Bowman's gland cells were seen in any group postoperatively. A degeneration in some neurons began to appear on the third day following the operation and reached its peak about one week after operation. No changes of the basal cells were seen on the third day. After one week, however, they changed into globose type and some of which showed mitotic activity. Within 2-4 weeks, the globose cells differentiated gradually into mature neurons. Four weeks after the operation, the regenerated olfactory epithelium appeared similar to that of the control group. The main feature of neuron degeneration was the increase in plasma electron density. In serious cases, the organelle systems disappeared and the nuclei were pyknotic. The results obtained from this experiment shows that after the sectioning of olfactory nerve, degeneration and regeneration processes of olfactory neuron will occur; and the regenerated neuron may possibly be developed from basal cells.