Ectoparasites and anthelmintics used to control external and internal parasites in livestock are largely excreted in manure in concentrations that are lethal or sub-lethal to beneficial organisms in the ecosystem. The objective of this study was to determine whether composting would reduce the concentration of ivermectin found in the manure of de-wormed horses. The effect of ivemectin on the composting process was also investigated. Manure and bedding from 60 horses at Oxley Equestrian Facility, Cornell University, Ithaca, NY de-wormed with 114 mg ivermectin paste each, was collected over a 3-day period in 2011 for composting. The presence of ivermectin did not appear to affect the composting process as shown by thermophilic temperatures and proper mass loss. There was a severe drop in ivermectin concentration in the manure/bedding mixture within the first few days of composting indicating exponential decay. This decay occurred at a rate of 1.8% per day with a half-life of 3.6 days. After 175 days of composting, ivermectin concentration decreased from an average of 1.59 mg/kg in the initial mixture to 0.6 mg/kg in the composted material and was not detected in the soil below the pile. However, potentially mineralizable nitrogen, a measure of microbial activity in soil, decreased in the soil on which the pile was built.