The plasma progesterone concentrations were measured in one group of tammars undergoing non-delayed oestrous cycles and pregnancy, and in another group undergoing pregnant and non-pregnant cycles which had been inhibited by lactation and initiated by removal of the pouch young. The basal concentrations of progesterone during lactation and for the first 5 days of the cycle were less than 200 pg/ml. In all animals there was a consistent peak of about 450 pg/ml lasting 1-2 days, on days 5-8 and this was followed by a return to the basal concentrations. From day 10 the concentration rose and remained at 500 pg/ml until the day of parturition and/or oestrus when it again returned to the basal concentration. There was no difference in pattern of the early peak associated with pregnancy but the peak concentration in the delayed pregnant cycles was just significantly higher than in the non-pregnant cycles of the same animals. These data do not support the hypothesis for the maternal recognition of early pregnancy in the tammar.