In this study we determined whether craniofacial development in rats could be influenced by an early temporary (3 d) nasal obstruction associated with forced oral breathing. The rats were killed at specific time points after surgery. Plasma samples were taken for biochemical analyses, and cephalometric measurements were performed. Shortly after nasal obstruction, the vertical nasomaxillary complex and the longitudinal skull base proved to be smaller in both sexes of test rats compared with controls. This was maintained in male rats but not in female rats. In female rats, only the longitudinal skull base remained somewhat shorter as the animals grew older. Reversible nasal obstruction was further associated with reduced dimensions of the olfactory bulbs lasting into adulthood and an initial decrease in lung weight. One day after implementing nasal obstruction, basal corticosterone levels had increased (by over 1,000%) and stayed at a high level in female rats. In male rats, however, the corticosterone level seemed to return to normal by day 90. Oral breathing was also associated with a lower level of thyroid hormone, especially at the shorter term intervals in both sexes. We conclude that a 3-d nasal obstruction period in young rats leads to long-term hormonal changes and to craniofacial structural adaptation