AbstractNeurogenesis and proliferation of olfactory receptor neurons (ORNs) in the olfactory epithelium (OE) are reduced in postnatal hypothyroid rats and upregulated following restoration of thyroid function, leading to compensatory growth and restitution of these deficits [Paternostro M. A. and Meisami E. (1993).Dev. Brain Res.76, 151–161; Paternostro M. A. and Meisami E. (1994).Dev. Brain Res.83, 151–162]. To investigate thyroid hormonal role on maturation of ORNs, serial sections of the septal OE from normal newborn, 25‐ and 90‐day‐old rats were immunostained for olfactory marker protein (OMP), a marker for mature ORNs, and compared with the same from age‐matched hypothyroid rats and those allowed to recover from thyroid deficiency at the time of weaning (day 25). The parameters studied were the localization and distribution of the OMP(+) cells within the OE and their density and total number. Hypothyroidism was induced by adding the reversible goitrogen propylthiouracil (PTU) to the rats' drinking water (1 g/l) from birth to days 25 or 90. Recovery from hypothyroidism was induced by withdrawal of PTU at day 25. The OMP(+) cells occupied a distinct, broad band in the normal rat OE, while in hypothyroid animal, this band was narrow and restricted to OE's apical zones. Recovery resulted in broadening of the OMP(+) cell band and normalized distribution of OMP(+) cells as evident in the 90‐day‐old recovery animals. In normal control rats, density of OMP(+) cells increased by 2.5‐ and 1.3‐fold during the suckling and post‐weaning period (days 25–90), while total numbers of these cells increased by 12‐ and 3‐fold, respectively, during the same age periods. Hypothyroidism decreased the growth in density by 25 and 30%, while total number of OMP(+) neurons were reduced by 40 and 70% in the 25‐ and 90‐day‐old animals, respectively. Withdrawal of PTU resulted in marked restoration of these deficits so that, at 90 days, the total number of OMP(+) cells were only 20% less than 90‐day‐old controls. These results indicate that thyroid hormones are essential for maturation of single ORNs and accretion of new mature ORNs in the OE of suckling and post‐weaning rat. Also, the process of maturation and the final number of mature ORNs show remarkable recovery from hypothyroid‐induced growth retardation.