Holstein cows received, subcutaneously 1 (1DESL, n=15) or 2 (2DESL, n=12) degradable implant containing 2.1mg of the GnRH agonist Deslorelin or no implant (CON, n=18) within 1.5 days postpartum (dpp). Previous pregnant (PPH) and non-pregnant (PNPH) uterine horns were determined by palpation per rectum. Cows were examined by ultrasonography at 8dpp, 15dpp, 22dpp, 29dpp, and 36dpp (S.E.=1 day) to record ovarian structures, cervical diameter, uterine horns cross-section and lumen diameters, myometrial and endometrial widths. Uterine tone was recorded before ultrasonography. Vaginoscopy was conducted just after ultrasonography for cervical discharge score. At 44dpp cows were inserted with a CIDR followed 7 days later by its removal and injection of PGF(2alpha) 8h later, followed by the Ovsynch 10 days after for timed artificial insemination (TAI). Plasma was analyzed for PGFM daily from parturition to 14dpp and for P(4) trice weekly until 44dpp. Additionally, strategic blood samples were collected during the synchronization protocol to determine whether estrous cyclicity was occurring and ovulation status before and after TAI, respectively. Cows in 1DESL and 2DESL groups had more class 1 follicles (P<0.01), less class 2 (P<0.01) and class 3 follicles (P<0.01) compared with CON. First increase of P(4), indicative of ovulation, occurred in CON (55.5%) cows at 28dpp (S.E.=9 days) and in 1DESL (13.3%) treated cows at 43dpp (S.E.=3). Plasma concentrations of P(4) were suppressed completely in all 2DESL-treated cows before initiation of estrous synchronization. Diameters of PPH (P<0.01), PNPH (P<0.01), uterine horn lumens (P<0.01) were less in the 1DESL and 2DESL groups with greater uterine tone (P=0.07). Frequency distribution of cervical discharge categories did not differ among groups. Proportion of cows with estrous cycles and having ovulations was less (P<0.01) in DESL implant cows compared with CON that was to a greater (P<0.01) extent in the 2DESL. Treatment with Deslorelin during postpartum (1) suppressed follicular development, (2) enhanced physical involution of the uterus, (3) increased tone of the uterine wall, (4) delayed first ovulation and reduced responsiveness to a synchronization of ovulation protocol. Future research should focus in GnRH agonist delivery systems to restrict duration of treatment to optimize uterine involution and avoid a prolonged period of anovulation.