Methylazoxymethanol (MAM) causes a reduced development of cerebellum and medulla oblongata in rats. To study the effect of external factors on growth of the spheno-occipital synchondrosis, four litters of rats each of five animals were treated with MAM within 12 h after birth and sacrificed after 30 days. A similar number od control rats were sacrificed at the time when their skull length was equal to that of the experimental rats. Microradiographical and histological investigations showed that the cranial base lordosis was more pronounced in the MAM rats than in the controls, and that the width of the spheno-occipital synchondrosis was reduced mainly due to reduction in the central zone.