Neural crest cells from the cranial region of the neural fold populate the outflow tract of the developing chick heart. Removal of this region of premigratory neural crest has been shown previously to result in a high percentage of conotruncal malformations. The present study was undertaken to define more precisely the regions of premigratory neural crest which are needed for normal conotruncal development. Various regions and lengths of premigratory cranial neural crest were ablated using microcautery. Three defects in conotruncal development were significantly correlated with the neural crest ablation. These were high ventricular septal defect, single outflow vessel originating from the right ventricle, and single outflow vessel overriding the ventricular septum.