Contents The study compared response to prostaglandin F2α (PG), synchrony of ovulation and pregnancy per AI (P/AI) in a 5- vs a 7-day Ovsynch + PRID protocol and investigated whether the initial GnRH affects P/AI in lactating dairy cows. Two hundred and seventy-six cows (500 inseminations) were assigned to one of four timed-AI (TAI) protocols: (i) PRID-7G; 100 μg GnRH im, and a progesterone-releasing intravaginal device (PRID) for 7 days. At PRID removal, PG (500 μg of cloprostenol) was given im. Cows received the second GnRH treatment at 60 h after PRID removal and TAI 12 h later. (ii) PRID-5G; as PRID-7G except the duration of PRID, treatment was 5 days and PG was given twice (12 h apart). (iii) PRID-7NoG; as PRID-7G except the initial GnRH, treatment was omitted. (iv) PRID-5NoG; as PRID-7NoG except the duration of PRID, treatment was 5 days. Response to treatments and pregnancy status at 32 and 60 days after TAI was determined by ultrasonography. The percentage of cows ovulating before TAI was greatest in PRID-7G (17.1%), and the percentage of cows that did not have luteal regression was greatest in PRID-5G (9.5%). The overall P/AI at 32 and 60 days did not differ among TAI protocols. However, during resynchronization, cows subjected to the 5-day protocols had greater (p
