In order to try to determine whether differentiated olfactory receptors turn over (die and are replaced by newly differentiated cells) during adult life, mice were injected with a single dose of ₃H‐thymidine at either 2 or 4 months of age and allowed to survive for up to 12 months; they were caged in a laminar flow unit to prevent rhinitis. Counts of labeled receptor cells detected autoradiographically after injection at 2 months of age revealed that, following an initial decrease from 1 to 3 months of survival, numbers of labeled cells remained approximately constant, at least up to 12 months of survival. Cells still labeled at 12 months of survival were confirmed as receptor cells by electron microscopic examination of reembedded sections. The hypothesis is suggested that in the absence of disease‐related destruction of the olfactory epithelium, most or all receptor cell turnover represents newly formed cells that fail to establish synapses with the olfactory bulb; fully differentiated receptor cells may be quite long‐lived.