Gross and microscopic anatomical investigations were carried out in 14 piglets aged from 4 to 66 days. True Blue (7-50 microliters) and Diamidino Yellow (7-50 microliters) were injected individually into 2 different cardiac sites (the right atrial ganglionated plexus, the inferior vena cava, inferior atrial ganglionated plexus, the right atrium or the right ventricle). Gross anatomy: Globular superior cervical and nodose ganglia, elongated stellate ganglia, multiple small middle cervical ganglia and multiple small mediastinal ganglia along the course of cardiopulmonary nerves were identified. Microscopic anatomy: Neurons innervating specific cardiac regions or intrinsic cardiac ganglionated plexuses were distributed relatively evenly among stellate (primarily in their cranial poles) and middle cervical ganglia bilaterally, fewer labeled neurons being located in the superior cervical and mediastinal ganglia bilaterally. Parasympathetic efferent preganglionic neurons associated with either intrinsic cardiac ganglionated plexus studied were identified primarily throughout the ventrolateral region (the external formation) of the nucleus ambiguus bilaterally. Labeled neurons were also identified throughout the right and left nodose ganglia. Individual neurons did not project axons to different cardiac regions, as no double-labeled neurons were identified. No correlation between age and the numbers and locations of labeled neurons was apparent. Thus, porcine sympathetic efferent neurons which innervate individual cardiac regions, including intrinsic cardiac ganglionated plexuses, lie scattered primarily throughout the right and left mediastinal and middle cervical ganglia as well as the cranial poles of stellate ganglia at birth, apparently changing little during the first 2 months of age. Porcine cardiac parasympathetic efferent preganglionic neurons are located primarily in the external formation of the nucleus ambiguus bilaterally at birth. The numbers of afferent cardiac neurons distributed throughout the nodose ganglia bilaterally also change little during that time. It is concluded that most of the autonomic neurons which innervate the heart are in place at birth.