We have studied the early development of subcortical projections to presumptive somatic sensory-motor areas of neocortex in the North American opossum Didelphis virginiana. The opossum is born in a very immature state, 12-13 days after conception, and climbs into an external pouch where it is available for experimental manipulation. Using the retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, we have obtained evidence that axons from the dorsal raphe and superior central nuclei, the substantia nigra, the locus coeruleus and the parabrachial nuclei reach presumptive somatic sensory-motor areas of neocortex by at least postnatal day (PND) 10. Axons showing serotonin-like immunoreactivity, presumably from the dorsal raphe and/or superior central nuclei, and axons containing tyrosine hydroxylase immunoreactivity, presumably from the substantia nigra and/or locus coeruleus, are present in the same areas at birth or shortly thereafter. Thalamic axons do not grow into comparable areas of neocortex until after PND 10. Such axons reach the subplate region of ventrolateral neocortex first and then proceed dorsomedially; by estimated PD (EPND) 21, they are present in presumptive layers I, V and VI, but they do not innervate an identified layer IV until EPND 48. The developmental sequences suggested by our study are compared with those reported for other species and are discussed in light of their importance in the formation of major sensory and motor circuits.