Activity of ivermectin, administered i.m. at the dosage rate of 200 .mu.g/kg of body wt, was evaluated in controlled tests against migrating larvae of S. vulgaris and adult T. axei in experimental infections in 6 ponies raised worm-free. Ponies were given 2190 or 2400 infective 3rd-stage larvae of S. vulgaris at 7 days before treatment and 22,000 or 22,750 infective 3rd-stage larvae of T. axei at 42 or 45 days before treatment. Three ponies were given ivermectin plus vehicle, and 3 ponies were given the vehicle only; the ponies were killed (euthanasia) 7 or 9 days after treatment. At necropsy, 4th-stage S. vulgaris larvae were not recovered from visceral arteries of the 3 ivermectin plus vehicle-treated ponies, but 21-40 larvae were recovered from each of the 3 vehicle-treated ponies. At necropsy, adult T. axei (140 specimens) were recovered from only 1 of the 3 ivermectin plus vehicle-treated ponies, but 4610-6410 specimens were found in each of the 3 vehicle-treated ponies. Toxicosis was not observed after treatment.