Metformin is used for the treatment of non-insulin dependent diabetes in humans. These conditions are pervasive and it is one of the most widely prescribed drugs worldwide. It has been studied in equines, administered orally at a dose of 30 mg/kg, for the treatment of conditions associated with hyperglycemia. In Thoroughbred racing horses in the U.S., metformin is a banned substance, and severe consequences result from a positive test. Additional studies are conducted to determine if significantly smaller doses, than those used clinically, produce detectable concentrations in the blood and urine. In this pilot study, 6 horses were administered metformin orally at doses ranging from 125 to 500 mg, once. Blood and urine samples were collected before administration and for predetermined time points from 1 to 12 hours after dosing. The concentrations of metformin in the samples were determined by LC/MS/MS. Although the bioavailability of metformin in horses has been reported to be less than 10%, the analyte was above the Limit of Quantitation in all blood and urine samples. The highest peak serum concentrations were above 26 ng/ml. Although the horses had been fasted for 12 hours before dosing, metformin absorption was slow, with peak concentrations, which were variable and not necessarily dose-dependent, occurring 2 hours after drug administration. Not surprisingly urine concentrations were even more variable with a peak concentration of 27 ng/ml in one horse administered 500 mg of metformin 4 hours after dosing. Given its reportedly poor bioavailability, these results are surprising. Therefore, additional studies are underway to determine serum and urine concentrations of even smaller doses and the persistence of those findings. The results of this current study suggests that exposure to crushed tablet residue or metformin excreted in urine from a human could produce the analytical findings observed recently in the U.S.