Few studies have examined thalamic development in the embryonic mammalian brain. For a better understanding of the development of the thalamus, the time sequence of the location of neurons in thalamic nuclei was examined in a developing marsupial embryo and fetus. To accomplish this, uptake of tritiated thymidine was examined in the gray short-tailed opossum (Monodelphis domestica) from 3 days before birth through 16 days after birth. Total neurogenesis was optimal 4 days after birth. Plots of the labeled cells revealed that thalamic and cortical development are almost concurrent, similar to the time course of eutherian mammals. A consistent time gradient of proliferation in dorsal thalamus was not found. Rather, sectors of the dorsal thalamus showed significant differences regardless of day of development. The dorsal sector showed more neurogenesis than the ventral sector, caudal more than rostral, and lateral more than the medial sector. An analysis of individual nuclei of dorsal thalamus showed no significant differences in amount of neurogenesis with day of injection.