We have used the retrograde transport of Fast blue (FB) to study the origins of supraspinal projections to the lumbar and cervical spinal cord at different stages of development in the Brazilian, short-tailed opossum, Monodelphis domestica. Monodelphis was chosen for study because its young are born in a very immature state, 14-15 days after copulation, making it possible to manipulate its nervous system in an embryonic state without intra-uterine surgery. When injections of FB were made into the lumbar cord at postnatal day (PD) 1, neurons were labeled within several areas of the reticular formation (the retroambiguus nucleus, the ventral and dorsal reticular nuclei of the medulla, the gigantocellular reticular nucleus, the lateral paragigantocellular reticular nucleus, and the pontine reticular nucleus), the presumptive coeruleus complex, and the lateral vestibular nucleus. In many cases, labeled neurons were also found within the caudal raphe and the presumptive interstitial nucleus of the medial longitudinal fasciculus. The results of immunocytochemical studies provided evidence for catecholaminergic and serotoninergic neurons in the brainstem at PD1 and for axons of both phenotypes in the spinal cord. By PD3, labeled neurons were found within the ventral gigantocellular and ventral pontine nuclei of the reticular formation, the spinal trigeminal nucleus, and the presumptive paraventricular nucleus of the hypothalamus. When injections were made at PD4, neurons were also labeled within the medial and inferior vestibular nuclei, the red nucleus, the mesencephalic nucleus of the trigeminal nerve, the presumptive nucleus of Edinger-Westphal and the lateral hypothalamus. By at least PD7, the pattern of supraspinal labeling was similar to that obtained at older ages and in the adult animal. When FB was injected into the cervical cord at PD1, neurons were labeled in all of the areas labeled by lumbar injections at the same age and in larger numbers. In addition, labeled neurons were found within the ventral gigantocellular and spinal trigeminal nuclei. When cervical injections were made at PD15, labeled neurons were found within the deep cerebellar nuclei and amygdala and by PD17 they were also present within the superior colliculus and cerebral cortex. In some cases, cortical labeling was present outside the areas labeled by comparable injections in adult animals.(ABSTRACT TRUNCATED AT 400 WORDS)