This paper describes optimization of the GC-MS method used in toxicological laboratories for trace analysis of opiates in biological matrices. Urine samples were extracted on SPE cartridges and derivatized by silylation. The precision (repeatability) of the method for determination of N-desmethyl metabolites of opiates was rather irreproducible when using silylation with N-methyl-N-(trimethylsilyl)trifluoracetamide (MSTFA) for derivatization. The results improved when MSTFA in a mixture with NH₄I and stabilized with 1,2-bis[(trimethylsilyl)sulfanyl]ethane was used. The GC-MS method in the SIM mode is suitable for detection and quantification of traces of specific opiates in urine with the limit of detection ca. 3 ng.ml^-1 and the limit of quantification 10 ng.ml^-1. The precision of the method (variation coefficient) obtained for individual analytes at concentration levels 10, 100 and 500 ng.ml^-1 ranged between 0.5 and 16.2 %. Standard calibration curves for individual opiates were linear between 0 and 500 ng.ml^-1 with correlation coefficients above 0.9937.