Adult vertebrate olfactory epithelia are unique in their continual sensory neuron turnover and replacement. This paper describes studies of various aspects of the death of these receptor neurons in unoperated rats and at 12 days and 7 weeks after unilateral ablation of the olfactory bulb, the receptor neuron synaptic target. Particular attention has been focused on the lifespan of the dying cells using tritiated thymidine/autoradiography to examine their "birthdates," We show that in control epithelia 25-30% of the degenerating (pyknotic) cells were located in the basal quarter of the epithelium, the location of the least mature sensory neurons and of neuronal stem cell proliferation. Birthdate analysis shows that 2-5% of the degenerating cells were dying within a day or less of their "birth." Thus, a sizeable proportion of these cells were dying precociously, before achieving full neuronal maturation. A further 65% of the dying cells occured in the middle half and 7% in the apical quarter of the epithelium, Following unilateral olfactory bulbectomy a two- to threefold increase in the number of degenerating cells occurred in ipsilateral versus contralateral tissue. This was maintained through the 7-week experimental period. A shift of 10-15% of the total degenerating cell numbers from the basal to middle region of the epithelium also occurred. Despite the increased degenerative activity ipsilaterally, the proportion of dying cells labeled autoradiographically remained the same on both sides at most labeling periods. However, a striking wave of enhanced cell death of 6- to 7-day-old neurons over the contralateral levels occurred ipsilaterally in both the 12-day and 7-week postbulbectomy animals. This age likely corresponds to initial bulbar target contact by differentiating receptor neurons. Our findings point to the existence of multiple levels of epigenetic regulation of degenerative activity within the olfactory epithelium.