Twelve groups of falcons, each containing three female gyrfalcon-peregrine falcon hybrids (Falco rusticolus x Falco peregrinus) were injected intramuscularly with a single dose of ivermectin ranging from 0.2 mg/kg to 11 mg/kg bodyweight, and a control group was injected with water. Doses of ivermectin between 0.2 and 5 mg/kg failed to produce clinical signs of illness in the birds. Four birds which received either 6, 7 or 8 mg/kg showed slight clinical signs, and all the birds receiving 9 to 11 mg/kg showed more or less severe clinical signs of anorexia, apathy and sedation. Slight changes in the mean plasma activities of aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase (AP) were detected in the group dosed with 5 mg/kg, and higher dosages caused marked changes in these enzymes as well as in the mean plasma activity of lactate dehydrogenase. The mean activity of AP decreased, and the activities of the other enzymes increased. A dosage of 2 to 3 mg/kg ivermectin is recommended as a safe and effective antiparasitic drug for falcons and it has been used successfully to treat infestations of Serratospiculum species.