Efficacies of hCG and GnRH with respect to follicular diameter in Cosynch protocol were investigated. Lactating cows (n=119) received injections of GnRH and PGF2 7 d apart. Norgestomet ear implant was inserted at GnRH administration and removed prior to PGF2 injections. Presence of the corpus luteum (CL) at GnRH and PGF2 injections, and diameter of preovulatory follicle [small (SPF) <8 mm or large (LPF) ≥8 mm] at PGF2 were determined with transrectal ultrasonography (USG). The animals were randomly treated with GnRH (0.01 mg; GPG, n=62) or hCG (1,500 IU; GPH, n=57) at timed artificial insemination (TAI) 56 h after injection of PGF2