On the day of the second PGF2administration, plasmaprogesterone concentrations were low for heifers treatedduring the early luteal phase compared to the late lutealphase (P < 0.05, Table 1). Thereafter, PGF2administrationcaused 100% luteolysis (plasma progesterone < 1 ng/mLwithin 48 h of administration) regardless of the luteal phaseon the day of administration. Similar concentrations ofplasma progesterone (P > 0.05) were recorded on the dayof synchronized estrus (D0) and on day 5 after estrus inheifers treated with PGF2during the early or late lutealphase