The effects of vitamin B12 (B12) and phosphorus (P) around mating on reproductive performance of ewes have not been investigated previously. Therefore, this study aimed to investigate the effect of injection of a solution (Catosin 10%) contained B12 and P on metabolic status and different aspects of reproductive per-formance of ewes. A total of sixty multiparous Afshari ewes were divided into two groups (n=30 ewes/group) based on age, body condition score (BCS) and body weight (BW). Animals were received either 5 mL of Catosin (B12+P) or 5 mL of sterile water (Control) subcutaneously every 7 days during 28 days before mating and until 7 days after mating. Each mL of Catosin contained 0.05 mg of cyanocobala-min and 100 mg of butaphosphan, which provided 17.3 mg of P in the form of [1-(butylamino)-1-methylethyl]-phosphonic acid. Two weeks after the start of the experiment, the estrus cycles of all ewes were synchronized using intravaginal 12-day CIDRs. Ewes receiving B12 + P had higher BW and BCS at day of CIDR removal and BCS at day 30 after mating (P<0.05). Plasma E2 and testosterone to P4 ratio at the day of CIDR removal tended to be lower in ewes receiving B12 + P (P=0.06). Injection of B12 + P did not influence plasma concentrations of minerals and metabolites (P>0.05), whereas, ewes receiving B12+P had higher plasma concentration of B12 at the day of CIDR removal and the day of estrus and mating (P<0.05). The results suggest that B12 + P injection around mating may be an effective strategy for improving repro-ductive performance of ewe. However, further researches need to re-evaluate and confirm these findings. Manuscript profile