The olfactory sensory neurons are unique in the vertebrate nervous system in that they are replaced following experimentally induced degeneration. Unilateral removal of the olfactory bulb in hamster results in degeneration of all mature receptor neurons followed by a neurogenesis and partial replacement of the receptor cell population. To determine if full recovery requires the presence of normal target tissue, a study of sensory neuron replacement was made following a nerve transection procedure, which leaves the olfactory bulb (target) intact. A comparison of quantitative measurements of cell number and thickness in the sensory epithelium showed that the presence of the target tissue alone did not result in improved recovery. One possible explanation is that complete recovery requires that axons of newly formed receptors must first re-establish synaptic contact with the olfactory bulb. To test this possibility, it will be necessary to include longer postoperative recovery times than those used in the present study.