Nasal mucosa including olfactory and respiratory mucosa prevent lower respiratory tract from being injured by various kinds of harmful irritants. This mucosa is continuously destructed by these harmful ones and then replaced by new cells regenerated from basal cells. But it has not been known until now what factors regulated the regeneration potentials in olfactory and respiratory mucosa. Recently some reports showed that some of neurotransmitters such as substance P and calcitonin gene related polypeptide (CGRP) released from capsaicin sensitive nerve has some neurotrophic action. Namely, these has been noticed to have an promoting effect on the regeneration of taste buds, grafted skin, intestinal microvilli and connective tissue cells. This study is to investigate whether the capsaicin sensitive nerve in nasal mucosa has some effect on the growth of olfactory mucosa. In this experiment, the effect of capsaicin administration on the change of olfactory epithelium and the growth of olfactory bulb were studied in 10 days old rats by determining microscopic morphology and mitotic activity of olfactory receptor cells, and measuring the weights of bulbs. This study showed that early postnatal capsaicin treatment resulted in degenerative morphological change and decreased mitotic activity of the olfactory epithelium and reduced weights of bulbs. In degenerated olfactory mucosa, normal appearing receptor cells, even supporing and basal cells in olfactory mucosa were not found. These results indicated that capsaicin sensitive nerve could play a role to control the growth of olfactory mucosa and bulbs.