This paper describes a procedure for the quantitative determination of alfentanil, dextromoramide, fentanyl, methadone, pentazocine, pethidine, phenoperidine and phencyclidine in human plasma. Some drug metabolites were included in the study. The procedure involves a simple alkaline extraction technique and capillary gas chromatography separation followed by a nitrogen-selective detection. Concentrations as low as 10 micrograms/l can be routinely evaluated, making this method suitable for use in overdose drug screening, therapeutic drug monitoring and pharmacokinetic studies.