Changes in milk protein gene expression and specific prolactin binding were quantified in mammary tissue from the tammar wallaby (Macropus eugenii) at different stages of lactation. The transition from early (phase 2) lactation to late (phase 3) lactation was characterized by the induction of the gene for late lactation protein, a novel whey protein. During the same period, the levels of beta-lactoglobulin and beta-casein gene expression increased, whereas there was no change in the levels of expression of alpha-lactalbumin and alpha-casein genes. Prolactin binding in the mammary gland doubled during the latter half of phase 2 of lactation but declined significantly during the transition to phase 3 of lactation. These changes in prolactin binding resulted from changes in the number of receptors and not from a change in the affinity of the receptor for prolactin. Treatment of membranes with concanavalin A increased the number of prolactin-binding sites by 40% in membranes from phase 2 mammary tissue but decreased binding by 40% in membranes from phase 3 tissue, indicating that significant changes had occurred in the membranes of cells during this period. The tammar wallaby can secrete phase 2 and phase 3 milk from adjacent mammary glands (asynchronous concurrent lactation) and the developmental changes in milk protein gene expression and prolactin binding observed during lactation were reflected in these individual glands. Taken collectively, these findings suggest that mammary development and milk secretion in the tammar wallaby are regulated by both endocrine and local (intramammary) mechanisms.