Peripheral plasma progestagens were quantified by a competitive protein-binding assay throughout pregnancy. The level of progesterone increased significantly between Days 0 and 8 (P less than 0-05) and again between Days 28 and 44 and reached a maximum on Day 64. Subsequently, it fell slowly until about Day 300 and then rose again during the last 30 days before reaching a minimum on the day after foaling. Very low concentrations of 17alpha-hydroxyprogesterone were found except between Days 40 and 120 and during the last 30 days before birth. Two unidentified compounds, one eluting slightly before and one after progesterone on Sephadex LH-20 columns, first appeared in plasma between Days 30 and 60 of gestation, and increased gradually to Day 300. Both compounds increased significantly (P less than 0-05) at about 5 days before birth and were not detectable on the day after foaling.