Growth characteristics of the tammar wallaby brain have been determined from 143 animals at various stages of growth from birth to adulthood, with particular emphasis on the 250 days of pouch life. The macroscopic anatomy of the brain is also described for pouch young at days 3,5,88 and 190 days after birth, and for the adult.The transition from the rapid to mature growth phases of each brain structure occurs between days 120 and 180 after birth, and coincides with the appearance of previously described developmental characters or behaviour. The allometric relationships of brain and body growth described in this marsupial resemble those described for primates.Because of the accessibility of the pouch young during the critical period of brain differentiation, this marsupial may provide a useful laboratory model for the study of brain development.