Butorphanol is an opioid used as analgesic in humans and other species. In horses, it can cause locomotor stimulation at low doses. This drug is not well chromatographed by GC and so, it is necessary to transform it into a more suitable compound, which can be done by derivatization. The derivatization of a drug is used to impart volatility, masking polar groups to improve the results in gas chromatographic analysis. We have evaluated N,O-bis(trimethylsilyl)- trifluoracetamide (BSTFA)+ 1% trimethylchlorsilane (TMCS) and N-methyl-N-trimethylsilil-trifluoroacetamide (MSTFA) as derivatizing reagents for butorphanol at 30, 60 and 80 degrees C during 15, 30 and 60 min. The effects of dilution of these reagents with toluene and the evaporation before the derivatization were tested. Both reagents can be used for butorphanol derivatization and analysis and the dilution and evaporation steps did not alter the final results. The best derivatization conditions were 15 min at 80 degrees C, although 60 degrees C, although 60 degrees C during 60 min were also suitable.