The anatomic and functional development of the suprachiasmatic nuclei (SCN) was studied in the gray short-tailed opossum, Monodelphis domestica. To provide a background for developmental studies, daily patterns of wheel-running behavior and SCN metabolic activity were studied in adult animals. Adult opossums manifested robust circadian rhythms in locomotor activity that were entrained by the daily light-dark cycle. The temporal course of SCN metabolic activity, studied by the 14C-labeled deoxyglucose autoradiographic technique, showed that the adult SCN were metabolically active throughout subjective day and relatively inactive during subjective night. SCN neurogenesis, determined using 3H-thymidine autoradiography, was active at postnatal day 3, the earliest age studied, and continued until postnatal day 7. It was not until postnatal day 16 that the SCN appeared as distinct nuclei by light microscopy. Study of the ontogeny of the daily rhythm in SCN metabolic activity showed that the nuclei were metabolically active during both day and night at postnatal day 16. On day 20, a clear day-night rhythm in SCN metabolic activity was first observed; the rhythm was even more pronounced on day 27. These results indicate that the gray short-tailed opossum has a functioning circadian timing system and that the anatomic and functional development of the SCN in this species occurs during the postnatal period.