The present study was conducted to investigate the effects of GnRH (Dalmeralin) treatment given on the day of mating on reproductive performance in Lohi sheep.  Seventy six ewes 3-5 year old, weighing 44 ± 0.4 Kg were put to rams at synchronized estrus using two intramuscular injections of 2 ml PGF_2α analogue (Dalmazin) 11 days apart. These animals were divided into two equal groups through random stratification by body weight and were given either saline (group 1) or 2 ml GnRH (group 11) on day of mating.  A total of 16 ewes were slaughtered on day 25 of pregnancy (eight ewes from each treatment group). GnRH administration on the day of mating increased (P<0.01) plasma progesterone concentration compared to control. There was a tendency for a higher non-return rate and a lower number of ewes returned to estrus in the GnRH treated ewes, but the difference between the two groups was non significant (P>0.05).  Pregnancy rate was higher in GnRH treated group (83.3%) than control (60%) group (P<0.05). The ewes in GnRH administered group had more twins (P<0.05) than those in control group (28 v 10). In conclusion, GnRH administration improved reproductive performance of ewes when administered on the day of mating, probably through its beneficial effect on embryo survival by enhancing luteal function.

Key words: GnRH, Ovarian function, Reproductive performance, Progesterone, Lohi sheep.