In this study, forty 2-2.5 months pregnant sheep naturally infected with gastrointestinal nematodes and lungworms were used. These sheep were divided into two equal groups according to the epgs per gram (EPG) present in the faeces and were kept indoors. In order to determine the genus and species and the rates of gastrointestinal nematodes, faecal cultures were prepared. To determine the species and the rates of lungworms, the Baerman-Wetzel method was used for larvae count and differentiation. Haemonchus sp., Ostertagia sp., Trichostrongylus sp., Bunostomum sp., Cooperia oncophora, Cooperia sp., and Nematodirus spathiger from Trichostrongyloidea and Dictyocaulus filaria, Muellerius capillaris, Cystocaulus ocreatus, and Protostrongylus sp. from Metastrongyloidea were observed in these examinations. Doramectin was administrated intramuscularly to sheep in the treatment group at a dosage of 0.2 mg/kg. On the 15th, 45th and 60th days, faecal samples were examined from each group for the presence of eggs of gastrointestinal nematodes and larvae of lungworms. No gastrointestinal nematode eggs or lungworm larvae were seen after each application, and so the efficacy of doramectin was evaluated as 100%. When the lambs born from the sheep in the treatment group were compared to the lambs born from the sheep in the control group the former had gained about 1,182 g more body weight by the end of the 3rd month (P>0.05). No adverse effects were observed in pregnant sheep or their lambs.