This study was carried out at Sakha Experimental Station, belonging to Animal Production Research Institute to evaluate the effects of oxytocin (                      OT) and prostaglandin (PG) administration within 6-12 h of calving on uterine involution and reproductive efficiency of lactating Friesian cows. Total of 15 multiparous Friesian cows with normal parturition averaging 400-650 kg live body weight, 2-7 parities and 2.5-3.5 body condition score were used in this study. Animals were divided into three groups, 5 animals in each. Animals in the 1^st group were i.m. injected with saline solution (control). Animals in the 2^nd and 3^rd groups were administrated with an i.m. injection of 50 IU OT and 3 ml PG (Estromate). Animals in all groups were administrated during the same injection schedule (6-12 h of calving). Routine examination of the genitalia per rectum was conducted once weekly to judge the uterine involution by ultrasonography to determine diameter of uterine horns (gravid and non-gravid), uterine body, length and diameter of cervix and vaginal length. Pregnancy was diagnosed by rectal palpation on day 45 post-insemination, days open (DO) and number of services per conception (NSC) were recorded. Results showed that diameter of non-gravid horns showed insignificantly slight reduction during post-partum period in each group. Diameter of gravid horns decreased (P<0.05) up to day 35 in control group, versus days 21 and 28 of post-partum in OT and PG groups, respectively. Diameter of the uterine body reduced (P<0.05) in OT and PG up to day 21 of post- partum as compared to day 28 in the control group, thereafter uterine horn diameter showed insignificant reduction in each group. Cervical length and diameter reduced (P<0.05) up to day 28 in the control group versus day 21 in OT and PG groups. The changes in vaginal length were not significant during post-partum in each group. Period elapsed from parturition to detectable symmetrical uterine horns was shorter in OT and PG groups (29.9 and 28.4 d) than in the control group (33.7 d), respectively. However, nearly complete cervical closure occurred earlier in treatment groups (26.8 d for OT and 25.5 d for PG groups) than in the control one (28.6 d), but the differences were not significant. It is of interest to note that resumption of vaginal length had the same duration, being on day 14 of post-partum period in all groups. Conception rate (CR) within 120 days of post-partum period was 66.7, 80 and 40% in OT, PG and control groups, respectively. Increasing CR of cows in PG group was associated with greater NSC (1.5 services) and longer DO (79.8 d) than those in the control group (1.0 service and 47.5 d, respectively). However, cows in OT group showed greater NSC and longer days open (2.5 services and 80.5 d., respectively). In conclusion, prostaglandin treatment within 6-12 h of calving (i.m. injection of 3 ml Estromate) as a purpose of stimulating involution had pronounced effect on the duration of uterine involution and improving reproductive performance of Friesian cows in term of conception rate.