The plasma progesterone concentrations during pregnancy and the oestrous cycle of the quokka were measured daily after each had been initiated by the removal of pouch young. Progesterone levels ranged from 0.6 ng/ml in the early stages of the oestrous cycle to about 2.5 ng/ml at the peak of the luteal phase. There was no significant difference between pregnant and non-pregnant states before the removal of the pouch young nor in the latter half of the cycle. However, the plasma progesterone concentration on days 3--4 after removal of the pouch young was significantly greater in pregnant animals when compared with non-pregnant animals at the same stage and also when compared with the levels before removal of young. This early peak in the concentration of progesterone in peripheral plasma is discussed in relation to the development of the previously dormant blastocyst.