Plasma progesterone levels decrease sharply within 12 hours after the initial treatment with Tiaprost and within 24 hours reach levels at about 3.18 nM (1 ng/mL). Over the following days, progesterone levels remain either slightly above (cows 1, 3, 5 and 6) or below (cow 2) 3.18 nM. In cow 4, levels remain for 17 days above 3.18 nM and drop below this level thereafter. At term, all cows show the lowest recorded progesterone levels (1 to 2 nM). Repeated luteolytic treatments with Tiaprost (cows 1 and 2) or estradio1 benzoate (cow 3) has no further influence on individual progesterone levels. Treatment with progesterone-releasing intravaginal device (PRID) does not significantly elevate progesterone plasma levels. Total estrogens in plasma remained in general unchanged.