The composition and volume of marsupial milk changes dramatically over the course of lactation to provide an appropriate diet for the tiny altricial neonate to complete its development after birth¹ . These changes occur in the mammary gland according to an intrinsic maternal program that is not influenced by the sucking regimen of the developing young² . Tammar wallaby, Macropus eugenii, pouch young transferred to mothers that are producing milk for an older young grow and develop at an accelerated rate. While it is plausible that growth factors in the milk such as IGF-I, which increases in concentration as lactation proceeds, are driving the observed growth acceleration, it is also possible that growth factors synthesized by the pouch young themselves may contribute to this rapid growth. We transferred day 60 post-partum tammar pouch young to weight-matched mothers at day 120 of lactation and compared their body parameters and tissue growth factor expression to age-matched controls at days 120 and 150 post-partum. Foster young grew at an accelerated rate and were significantly heavier at days 120 and 150 as demonstrated previously. Total liver weight was significantly higher than controls, but not as a percentage of total body mass (PBM). Both total and PBM fat deposition was significantly higher in foster young. Importantly, IGF-I mRNA expression was significantly higher (p<0.05) in the liver of foster young at 120 days, but only trending higher (p<0.07) by 150 days post-partum. These data suggest that growth factor expression in the pouch young is affected by nutritional manipulation and may be contributing to the accelerated growth observed in forward fostered young.