Total unilateral bulbectomy induces degeneration of the mature olfactory neurons and disappearance of the olfactory marker protein from the primary sensory pathway. Owing to the presence of a neurogenetic matrix in the neuroepithelium, reconstitution of a new population of neuronal elements occurs. In this experiment, connections of the regrown olfactory axons with the spared forebrain are barred by the formation of scar tissue. In spite of the absence of a target, new neurons differentiate and produce olfactory marker protein.