Our objective was to determine the effects of a single treatment of human chorionic gonadotropin (hCG )or GnRH during early diestrus on estrous cycle length, expression of estrus and fertility in lactating dairy cows. Lactating Holstein (n = 365) and Jersey (n = 212) cows detected in estrus by an automated activity monitoring (AAM) system between 27 and 50 DIM were randomly assigned to receive 1 of 3 treatments d 5 to 7 of the estrous cycle: hCG (3.300 IU i.m.; n = 200), GnRH (86 µg gonadorelin i.m.; n = 195), or control (no treatment; CON; n = 182). Ovaries were scanned with ultrasound in a subgroup of cows (n = 236) on the day of treatment and 3–4 d later to determine ovulation. Estrus was detected following treatment by an AAM during 32 d after pre-treatment estrus, and peak activity index was recorded. A random subgroup of cows observed in estrus received first AI 51 to 80 DIM (n = 342). Pregnancy diagnoses were performed by ultrasound at 37 ± 3 d post-AI. Data were analyzed by ANOVA and logistic regression using proc MIXED and GLIMMIX of SAS. The model included fixed effects of treatment, parity and the interaction of treatment × parity. Cows treated with hCG had higher (P < 0.05) ovulation rate than GnRH (85 vs. 73%). The proportion of primiparous cows returning to estrus was greater (P < 0.05) for hCG (93%) and tended to be greater (P = 0.06) for GnRH (93%) than CON (83%). However, hCG (73%) and GnRH (76%) decreased (P < 0.05) the percentage of multiparous cows returning to estrus than CON (87%). Cows treated with hCG had longer (P < 0.01) estrous cycle length (23.9 d) than CON (22.2 d) and GnRH (22.2 d). Percentage of cows with high peak activity (index ≥ 80) did not differ between groups (CON: 93.4%, GnRH: 96.2%, and hCG: 93.7%). Primiparous cows pregnancy per AI (P/AI) was greater (P = 0.05) and tended (P = 0.09) to be greater for hCG (53.1%) than CON (33.3%) and GnRH (36.0%), respectively. Treatment did not affect (P > 0.10) P/AI in multiparous cows (CON: 38.5%; GnRH: 38.1%; hCG: 32.8%). In summary, inducing the formation of an accessory corpus luteum on d 5 to 7 of the estrous cycle with hCG increased the likelihood of primiparous cows exhibiting estrus within 32 d and fertility of primiparous cows bred following estrus compared with controls.