Nerve fibres reactive to acetyl-thiocholine, and tissues showing catecholamine fluorescence were examined in the pulmonary trunk, ductus arteriosus and aorta of 28 pig fetuses between 31 and 113 days of gestation (term = 114 +/- 1 days). Eight additional fetuses, which had been decapitated in utero at 40-43 days, were also studied at ages between 51 and 114 days of gestation. Spherical micro-networks of nervous tissue reactive to acetyl-thiocholine are present in the adventitia on the cranial aspect of the pulmonary trunk and ductus arteriosus, between the aorta and pulmonary trunk, and on the caudal aspects of the pulmonary trunk and the pulmonary arteries. These fibres invest spherical clusters of catecholamine containing cells which are well supplied with blood vessels. Nerve fibres which fluoresce are also found in association with these cells. Decapitation in utero does not appear to affect the distribution of morphology of these structures. The observations show that structures are present in the major arteries of the fetal pig which may act as sensory receptors, and that these structures are unaffected by chronic vagotomy of the fetus produced by decapitation early in gestation.