The efficacy of doramectin (Dectomax, Pfizer) and moxidectin (Cydectin, Abfar) against gastrointestinal nematode infection was evaluated in cows which were naturally infected with these nematodes. For that purpose over 100 cows' faeces were tested for gastrointestinal nematodes with the saturated salt solution method. Infected cows were selected according to the McMaster criteria. To establish whether the cows were infected with gastrointestinal nematodes, their coprocultures were screened for the presence of Ostertagia, Haemonchus, Nematodirus and Trichostrongylus species. Thirty cows which had acquired nematodes naturally were allocated to three groups of 10. The cows in the first and second group were administered subcutaneous 1% doramectin and moxidectin in injectable form at 0.2 mg/kg, respectively. The third group was separated as a control. Faecal samples from animals were examined one week before the start of treatment and on days 0, 7 and 14 of treatment. One and two weeks after the treatment, the examination of faecal samples from both treated groups showed that doramectin and moxidectin were 100% effective against gastrointestinal nematode infection. In contrast, the egg numbers in the control group were variable but not lower.