Experimentally induced infections of Parascaris equorum in worm-free pony foals required 14 to 17 days for migration of the larvae through the liver and lungs, and 79 to 110 days to become gametogenically functional. Treatment of experimentally infected or naturally exposed foals during the parenteral phase of development, using levamisole at 8 mg/kg, a mixture of levamisole at 8 mg/kg plus piperazine at 88 mg base equivalent/kg, or dl-tetramisole at 10 mg/kg, was quite efficacious in (1) reducing the number of P. equorum larvae recovered from the small intestines of the foals at necropsy, or (2)delaying the appearance of ascarid eggs in the feces of treated foals beyond the time period observed for the untreated foals. Three formulations of dichlorvos at doses of 10, 20, 30, or 40 mg/kg, and 2 formulations of trichlorfon at 20 or 40 mg/kg, were not effective when treatment was given on or before the 14th day after infective eggs were administered. Treatment with a gel formulation of dichlorvos at 20 mg/kg 17 to 28 days after experimental infection was 100% effective in removing ascarid larvae from the small intestines of poly foals.