The 4 mammary glands of a nulliparous tammar wallaby differentiate during the 27-day pregnancy in preparation for lactation. Alveoli increase in size and number in the first half of gestation, and this increase continues slowly to term. At or soon after parturition the alveolar lumina enlarge as the gland to which the young is attached begins to secrete. The connective tissue stroma of the gland is progressively replaced throughout pregnancy and lactation with glandular tissue. Gross dimensions of the gland and alveolar size increase markedly in later lactation as the gland reaches maximum production. After parturition the other 3 glands simultaneously regress to a quiescent state until the next pregnancy. After weaning (Days 320-450 of lactation) the lactating gland involutes. This may occur concurrently with redifferentiation of the other glands for a new lactation. This pattern of differentiation is similar to that of the red kangaroo, and probably all macropodids.