Following closure of the last remaining equine slaughter plants in the United States in 2007 there is an increased need for equine carcass disposal. Based upon their knowledge of the equine industry, the authors estimate that approximately 200,000 horses must be removed from population each year. Disposal options for carcasses include burial, landfill, incineration, composting, or rendering. The most attractive option, for reasons discussed in the paper, is composting, which has been successfully implemented in a variety of environments and situations. The objective of this study was to determine if the euthanasia drug, sodium pentobarbital, remains in compost material containing carcasses of euthanized equines. Compost piles consisted of horse manure, wood shavings and waste hay. A licensed practicing veterinarian humanely euihanized each horse with a sodium pentobarbital solution injected into the jugular vein. The carcass was then deposited on the bed of waste hay. Two continuous recording electronic thermometers (HOBO units) were placed on the carcass, and the aircass was then completely covered with a layer of composting material. Pile temperatures were obtained continuously by means of both HOBO units and manually once a week by long-stem thermometers. The trial lasted 328 d, with individual piles mechanically aerated at about d 90 and 180. Core samples of piles were obtained at d 10, 14, 90 and 180. The pile samples were random grab samples taken from three different locations in the compost pile and then compiled into one sample to be sent off to the laboratory for analysis. Visual observations of all piles were conducted at time of pile aeration and carcass degradation scores were assigned. Compost samples were analyzed for sodium pentobarbital by the Centralia Animal Disease Laboratory (Centralia, Illinois). The presence of sodium pentobarbital was confirmed in all pile samples at d 90 and 180. At d 90 and 180 the pentobarbital concentrations ranged from 0.008 to 3.16 ppm. This data confirms the presence of sodium pentobarbital in compost piles containing carcasses of euthanized equines after 6 months of composting.