It has been proposed that the cervical flexure of vertebrate embryos arises from the normal morphogenesis of the heart. This hypothesis is based on experiments in which the heart tube is removed or disrupted in early chick embryos. It has been reported that, in normal atmosphere, these embryos continued normal morphogenesis except for cervical flexure formation. In the present study, we performed similar experiments. In contrast to previous work, however, only one set of our heart-deprived chick embryos was reincubated in normal air. The other sets were reincubated in oxygen-enriched air. Under normoxia, heart removal resulted not only in prevention of the cervical flexure, but also in mesenchymal defects, and in a remarkable hypoplasia of the craniocervical region. Under hyperoxia, heart-deprived embryos developed no severe mesenchymal defects and the growth of the upper body portion was more normal, with the hypoplasia confined to the cranial region. The formation of the cervical flexure was now normalized. These results show that cervical flexure formation is not directly dependent on normal morphogenesis of the heart, but does depend on a sufficient oxygen supply to the cervical region. During early development, the cranio-cervical region of a chick embryo is more sensitive to circulatory failure than the trunk.