Continuous replacement of sensory neurons is a normal process in the olfactory system of adult vertebrates. The capacity for replacement following experimentally induced degeneration of sensory neurons makes this system ideal for the study of the dynamics of neural populations. A quantitative analysis was made to determine the time course and degree of cell replacement in the olfactory epithelium following unilateral bulbectomy in adult hamsters. Histological measurements of number of cells and epithelial thickness were made for up to 194 days postoperatively. Results for each experimental animal were expressed as a percentage of the contralateral control side. There was an immediate degeneration of cells, the number decreasing to 39% by day 4. During days 4--15 new growth resulted in an increase in cell number, which was maintained at a level of 60--70% through day 194. Epithelial thickness decreased to 60--70% during the degeneration period, but there was no recovery during subsequent days 4--194. Analysis of epithelial cells by cell type (supporting, receptor, and basal cells) showed that changes in cell numbers were limited to the neural cell populations (receptor and basal). This study confirms that olfactory sensory neurons are capable of replacement following degeneration in spite of the absence of normal target tissue. However, the observed recovery does not reach control levels and the functional capacity of replacement neurons requires further study.