Objectives were to evaluate concentrations of progesterone (P4), ovulation incidence, fertility, and insemination pattern of primiparous dairy cows treated with GnRH before a presynchronization protocol during summer heat stress. At 60 ± 3 d postpartum, primiparous cows (n = 1,352) from 3 dairies were assigned randomly to 2 treatments (d 0): receiving GnRH (Gpresynch) or no GnRH (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 a 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 1 disease event occurred before AI. Pregnancy diagnosis was conducted 36 ± 3 d after AI by transrectal ultrasonography. In a subgroup of cows, examinations of ovarian structures were performed on d −14, 0, and 7 (n = 161) and blood samples were collected on d 0 and 7 to determine concentration of P4 (n = 167). Logistic regression analysis was used for dichotomous outcomes and continuous variables were analyzed by ANOVA. The rate at which cows were inseminated was analyzed using the Cox’s proportional hazard model. Ovulation risk to GnRH treatment was greater (P < 0.01) for Gpresynch than control cows. Percentage of cows that had a corpus luteum (CL) and concentration of P4 ≥ 1 ng/mL on d 7 was (P < 0.01) greater for Gpresynch cows. Concentration of P4 on d 0 (P = 0.91) and d 7 (P = 0.31) did not differ between treatments. An interaction between treatment and disease tended (P = 0.08) to affect P4 on d 7 because diseased cows treated with GnRH had greater P4 than diseased control cows. Controls tended (P = 0.06) to have a faster insemination rate than Gpresynch cows [adjusted hazard ratio = 1.12 (95% CI = 1.00, 1.26)]. In addition, pregnancy per AI did not differ (P = 0.25) between treatments. Although GnRH treatment increased the percentage of cows with a CL, no difference in subsequent fertility was detected.