Livestock in confined animal feeding operations in Canada are routinely administered antimicrobials (antibiotics), therapeutically for disease treatment and prevention, as well as sub-therapeutically for growth promotion. Up to 75% or more of administered doses may be subsequently excreted in feces and urine.  In beef cattle production, solid manure is removed from feedlot pens and frequently stored in stockpiles prior to land application.  Although veterinary antimicrobials have been shown to dissipate during aerated windrow composting of beef cattle manure, it is unclear whether they dissipate to the same extent during stockpiling, or whether the dissipation is dependent upon the location within the stockpile.  A study was conducted at Agriculture and Agri-Food Canada Research Centre, Lethbridge, AB in fall 2010 to investigate the dissipation in stockpiled manure of three antimicrobials commonly used in beef cattle production  Beef cattle (Bos taurus) were administered (1) 44 mg of chlortetracycline kg^-1 feed (dry-weight), (2) 44 mg of chlortetracycline + 44 mg of sulfamethazine kg^-1 feed, (3) 11 mg of tylosin kg^-1 feed and (4) no antimicrobials (control) in the research feedlot situated on the Research Centre.  Manure was cleaned from feedlot pens and placed in stockpiles (2 replicates per treatment = 8 stockpiles). Manure samples were collected periodically from three locations (top, center and bottom) within the stockpiles over a period of 140 d and analyzed for concentrations of chloretetracycline, sulfamethazine and tylosin using LC/MS/MS analysis. The concentration data will be used to investigate the dissipation kinetics of the three antimicrobials during the stockpiling process.