The effect of oxygen on the growth of the mandible was studied in 40 Long-Evans/Turku rats. Pregnant rats were exposed to increased oxygen tension for 16 days from 7 to 23 days of gestation. After parturition the young rats were kept under normal laboratory conditions until the age of 50 days and then killed for macroscopic examination. Fifteen rats were exposed to increased oxygen tension from 15 to 50 days postnatally and then killed. The control animals were kept under normal laboratory conditions throughout. A prenatal increase in oxygen tension had no marked effects on the dimensions of the mandible at the age of 50 days, although the height of the mandible was increased when the dimensions were corrected for body weight. A postnatal increase in oxygen tension reduced the length of the mandible but increased its height. Although low oxygen tension has been thought to favor cartilage formation, and although function is considered necessary for the growth of the mandibular condyle, it seems that the latter process is stimulated by increased oxygen tension and reduced by loading.