The aim of the study was to determine the effectiveness of the drug “Iversan” in nematodoses of the gastrointestinal tract of horses. three groups of animals were formed in the horse club “Prime-Horse” in the Lomonosov district of the Leningrad Region. Three groups were formed (two experimental and one control) from animals in which helminth eggs of the Strongylida, Parascarisеquorum, Oxyurisequi and larvae of the Delafondia, Alfortia, Strongylus, Trichonema, Strongyloides were found. Animals from group 1 were given Iversan at a dose of 1 ml/200 kg of animal weight (200 microgram ivermectin/1 kg) individually, orally with water from a syringe along the toothless edge on the tongue root. The horses in group 2 were given a preparation with food at the rate of 1 ml of the preparation/200 kg. The horses of the control group 3 were not given the Iversan. After 5 and 10 days after giving the drug, coprological studies were performed. In horses of the experimental groups, eggs and larvae of nematodes of the gastrointestinal tract were not isolated. In feces samples of horses in the control group who did not receive therapy, an increase in the intensity of invasion was observed. It was found that the drug “Iversan”, containing in 1.0 ml 40.0 mg of ivermectin, used orally, once has a pronounced therapeutic effect in nematodoses of horses