The current study aimed at evaluating the effect of GnRH treatment, during different intervals from calving (14, 21 or 28 day-postpartum) on reproductive efficiency, milk production and some hematological parameters of lactating Friesian cows. A total of 35 lactating Friesian cows at 7 day-postpartum (PP), ranging from 4-7 years of age, 2^nd - 4^th parity and 450-550 kg LBW was divided into four groups. Cows in the 1^st group (n=8) were injected with saline solution (0.9 % NaCl) on day 14-PP. However, cows in the 2^nd (n=9), 3^rd (n=10) and 4^th (n=8) groups were injected with GnRH on days 14, 21 and 28, respectively. Cows in all groups were observed for heat signs and were artificially inseminated by frozen semen when they were in heat. Pregnancy was confirmed by rectal palpation on d 60 post first artificial insemination. Blood samples were collected and count of red blood cells (RBC), packed cell volume (PCV %) and haemoglobin concentration (Hb) were performed in whole blood collected for 7 week-PP. The following traits were calculated:postpartum first oestrus interval (PPFOI), treatment to first oestrus interval (TFSI), days open (DO), service period length (SPL), number of services per conception (NSC), calving interval (CI) and conception rate (%). Also, average daily milk yield was recorded during the first 12 wk of lactation period. Results revealed that cows in G2 showed the highest CR (44.5%) within ≤120 day-PP as compared to 37.5% in G1.  Within >120 day-PP, cows in G3 showed the highest CR (70%). The effect of GnRH treatment on all reproductive measurements of Friesian cows having ≤120 day-PP was not significant. For Friesian cows having >120 days open, the effect of GnRH treatment on reproductive measurements was significant (P<0.05) only on CI, being longer in G2 (509.4 d) and G4 (514.4 d) than in G1 (461.8 d) and G2 (471.4 d). Within the whole postpartum period, cows in different experimental groups showed inconsistent trend and insignificant differences in all reproductive measurements. Average daily milk yield for 12 wk lactation of cows having ≤120 DO was not significantly affected by GnRH treatment. However, average daily milk yield of cows having >120 DO was lower (P<0.05) in G2 than in G1. Count of RBC and PCV percent were reduced (P<0.05) in G2 as compared to G1, while RBC count decreased (P<0.05) and PCV increased (P<0.05). The differences in all haematological parameters studied in G3 or in Hb concentration of all treated groups were not significant as compared to G1. The current study could conclude that injection with GnRH during different postpartum periods especially on 14 day-PP may help in acceleration of lactating cows to resume their ovarian activity and increasing conception rate during 120 day-PP.