Corpora lutea were obtained from wallabies at different stages of pregnancy, following removal of pouch young to initiate embryonic development. Progesterone was present at a concentration of 11.3 ng/mg in quiescent corpora lutea from lactating animals. Progesterone values rose with increasing corpus luteum wt to reach a maximum of 40-50 ng/mg in corpora lutea weighing 50-60 mg. Total progesterone reached a maximum of 1500 .+-. 300 (Standard error of the mean) ng at days 21-23 after removal of pouch young, but fell markedly at days 24 and 25 (900 .+-. 150 ng) immediately before parturition.