Cardiac troponin I (cTnl) has proven to be a highly specific and sensitive marker for myocardial cellular damage in many mammalian species. The structure of cTnl is highly conserved across species, and assays for human cTnl (including the one used in the current study) have been validated in the dog. Blood concentrations of cTnl rise rapidly after cardiomyocyte damage, and assay of cTnl potentially may be valuable in many clinical diseases. The purpose of this study was to establish the normal range of cTnl in heparinized plasma of dogs and cats. Forty one clinically normal dogs and 21 cats were included in the study. One to 3 milliliters of blood were collected by venipuncture into lithium heparin vacutainers for analysis of cTnl (Stratus® CS). The range of plasma cTnl concentrations in dogs was < 0.03 to 0.07 ng/mL with a mean of 0.02 ng/mL, with the upper tolerance limit (0.07 ng/mL) at the 90th percentile with 95% confidence. In cats, the range was < 0.03 to 0.16 ng/mL with a mean of 0.04 ng/mL, and the upper tolerance limit (0.16 ng/mL) at the 90th percentile as well with 90% confidence. This study establishes preliminary normal ranges of plasma cTnl in normal dogs and cats for comparison to dogs and cats with myocardial injury or disease.