Abstract. Effects of selenium on certain indicators of sheep reproductive success and on Se level in the sheep's blood serum were studied in 2001–2002 on 100 Polish merino sheep ewes kept in a Western Pomeranian sheep farm. The 100 ewes were picked out on random and divided into two equal groups: treatment and control. The blood samples for serum Se assays were collected prior to administration of sodium selenide (before servicing) and 7 days after the treatment. The study was aimed at determining Se effects on the reproductive success (fecundity) of ewes and on selected indicators of lamb utility in a Se-deficient area. Se administration prior to servicing enhanced the herd’s reproductive success. The treatment group’s litter size was by 0.38 lamb per ewe higher than that of the control. The control group lamb body weight was lower than that of the treatment group by an average of 0.23 kg on birth and 0.31 kg 33 days later.