Progesterone concentrations were measured in milk and blood plasma for 15 mares throughout a normal estrous cycle and early pregnancy to determine the feasibility of utilizing progesterone in milk as an indicator of pregnancy. Samples were obtained daily from foaling until diagnosis of pregnancy by rectal examination at 30 to 35 days of gestation. Progesterone in milk and blood plasma was quantified by radioimmunoassay. Mean progesterone concentrations (+/- SE) in plasma from foaling to foal heat and during estrus, luteal phase and pregnancy were .51 +/- .09 ng/ml, .53 +/- .08 ng/ml, 3.88 +/- .26 ng/ml and 4.22 +/- .09 ng/ml, respectively; progesterone concentrations in milk during the same periods were 1.57 +/- .16 ng/ml, 1.41 +/- .06 ng/ml, 4.26 +/- .21 ng/ml and 4.14 +/- .09 ng/ml respectively. Progesterone concentrations in milk were higher than concentrations in plasma only from foaling to foal heat and during estrus. Days of the estrous cycle on which milk and (or) blood plasma progesterone concentrations could be used to detect pregnancy were determined. Progesterone concentrations in milk from seven of the mares on each day from day 1 to 25 of a normal estrous cycle (nonpregnant) were compared to progesterone concentrations in milk from the same seven mares on days 1 to 25 of a subsequent fertile estrous cycle (pregnancy). (The first day that mares refused to accept the stallion respectively.) Identical comparisons were made of progesterone concentrations in blood plasma. Significantly lower progesterone concentrations were observed in milk from nonpregnant mares from days 17 to 22 and in blood plasma from nonpregnant mares from days 15 to 22.