To investigate the role of autonomic receptors in the closure of the ductus arteriosus, full term guinea-pig fetuses were treated with phenoxybenzamine, delivered by Caesarean section, and sacrificed 10 min later by immersion in liquid nitrogen. Semi-serial cryostat sections of the ductus revealed a significantly greater inner diameter in the phenoxybenzamine-treated animals as compared to saline-injected controls. Combined with more specific antagonists, the whole-body freezing technique could be useful to characterize the nature of the receptors involved in the closure of the ductus arteriosus.