Objectives of the experiment were to evaluate concentrations of progesterone (P4), ovulation incidence, fertility, and insemination pattern of multiparous dairy cows treated with GnRH before a presynchronization protocol during summer heat stress. At 49 ± 3 d postpartum, multiparous cows (n = 1,941) from 3 dairies were assigned randomly to 2 treatments (d 0): receiving GnRH (n = 965; Gpresynch) or no GnRH (n = 976; Control). Cows were presynchronized with 2 injections of prostaglandin F_2α (PGF) given 14 d apart beginning on d 7. Cows detected in estrus based on tail paint removal were inseminated. Cows not detected in estrus were enrolled in the Cosynch-72 protocol on d 35 (GnRH on d 35 and 45; PGF on d 42; timed AI on d 45). Incidence of uterine health disorders and mastitis recorded by farm personnel were collected. Cows were considered diseased if one disease event occurred before AI. Pregnancy diagnosis was conducted 36 ± 3 d after AI by transrectal ultrasonography. In a subgroup of cows, ultrasonography examinations of ovarian structures were performed on d −14, 0, and 7 (n = 351), and blood samples were collected on d 0 and 7 to determine P4 concentrations (n = 361). Logistic regression analysis was used for dichotomous outcomes and continuous variables were analyzed by ANOVA. The rate at which cows were inseminated or became pregnant was analyzed using the Cox’s proportional hazard model. Ovulation risk to GnRH treatment was (P < 0.01) greater for Gpresynch than control cows. Percentage of cows that had a corpus luteum (P = 0.01) and P4 concentration ≥1 ng/mL at first PGF (P = 0.03) was greater for Gpresynch than control cows. Concentrations of P4 on d 0 (P = 0.30) and d 7 (P = 0.54) did not differ between treatments. Furthermore, insemination rate did not (P = 0.52) differ between treatments. In contrast, control cows tended (P = 0.07) to become pregnant at a slower rate than Gpresynch cows [AHR = 0.81 (95% CI = 0.64, 1.02)]. Treatment with GnRH before a presynchronization protocol using PGF injections increased the percentage of cows bearing a CL at first PGF, but did not improve pregnancy per AI.