Tenoxicam (Mobiflex) was administered orally to four standardbred mares at a dose of 200 mg. Elimination profiles of tenoxicam and hydroxytenoxicam were generated based on quantitation of these analytes in urine and serum by liquid chromatography (LC) with ultraviolet detection. Tenoxicam was confirmed by LC-tandem mass spectrometry daughter ion mass spectra in the last postadministration sample in which tenoxicam was detected. The tenoxicam and hydroxytenoxicam urinary elimination profiles had the same shape for the same horse; however, each horse was significantly different from the others. One horse (Horse 15) showed a much broader and flatter elimination profile than the other horses. Each horse had a peak in analyte concentration at different collection times. The latest detection for both tenoxicam and hydroxytenoxicam was 29-31 h for all horses. The urinary tenoxicam limit of detection (LOD) and limit of quantitation (LOQ) were 0.3 and 0.4 microg/mL, respectively. The urinary hydroxytenoxicam LOD and LOQ were 0.6 and 0.8 microg/mL, respectively. Hydroxytenoxicam was found to be completely conjugated and tenoxicam completely unconjugated in equine urine. Serum elimination profiles of tenoxicam were measured to 120 h postadministration. Hydroxytenoxicam was not detected in postadministration serum. The last serum tenoxicam detection was at the 24-h collection time for all horses. The peak average concentration was 434.5 ng/mL at 3 h. The serum tenoxicam LOD and LOQ were 5.7 and 7.3 ng/mL.