The aim of the present study was to evaluate analytical performance using total error criteria of a commercial enzyme immunometric assay for the determination of endogenous canine thyrotropin (TSH). The allowable total error for this assay (22.6%) was estimated using previously reported data on biological variation. Inaccuracy and imprecision of the assay (0% and 5.7% for the low control material; 6.8% and 3.0% for the high control material) were estimated by measuring the same lot of control material for 21 consecutive weeks, during which time the assay was considered stable and in control Analytical performance was assessed using a MEDx chart, a graphical tool for comparing inaccuracy and imprecision, with an analytical quality requirement stated in the form of allowable total error. The results of the present study showed that the canine TSH assay had good to excellent analytical performance.