An analytical procedure to screen butorphanol in horse race urine using ELISA kits and its confirmation by GC-MS is described. Urine samples (5 ml) were subjected to enzymatic hydrolysis and extracted by solid-phase extraction. The residues were then evaporated, derivatized and injected into the GC-MS system. The ELISA test (20 microl of sample) was able to detect butorphanol up to 104 h after the intramuscular administration of 8 mg of Torbugesic, and the GC-MS method detected the drug up to 24 h in FULL SCAN or 31 h in the SIM mode. Validation of the GC-MS method in the SIM mode using nalbuphine as internal standard included linearity studies (10-250 ng/ml), recovery (+/-100%), intra-assay (4.1-14.9%) and inter-assay (9.3-45.1%) precision, stability (10 days), limit of detection (10 ng/ml) and limit of quantitation (20 ng/ml).