The total number of neurosensory cells in the mouse vomeronasal organ was estimated during postnatal development by counting the cell density and measuring the total volume of the neurosensory cell layer. There is a 43% increase in neurosensory cell number between 1 and 4 months of age, followed by a 21% fall in cell number between 4 and 8 months. There is no further significant change in cell number between 8 and 18 months of age. Cell division was shown to be occurring in the vomeronasal organ of animals at 7 months of age by labelling dividing cells with [3H]thymidine continuously administered by means of implanted 'osmotic pumps'. At least 1 in 6 cells were labelled by 12 days of thymidine administration, indicating a turnover time of 2--3 months for the whole epithelium. This raises the general problem of how a fixed central nervous system accommodates a changing peripheral input.