Remifentanil (G187084) is a phenylaminopiperidine derivative of the fentanyl type with potent analgesic activity. The compound has an N-substituted labile methyl ester which is highly susceptible to chemical and enzymatic hydrolysis resulting in a short half-life for the drug. A sensitive capillary GC-HRMS-SIM method for the determination of remifentanil in blood has been developed and progressively revalidated in response to pharmacokinetic needs. The method relies on immediate precipitation of blood proteins with acetonitrile to stabilize the drug, followed by liquid-liquid extraction with methylene chloride. Collection tubes are pre-spiked with tetra-deuterated remifentanil as an internal standard to correct for variations in recovery and stability between samples. An initial method was developed on a Hewlett-Packard GC-MSD and had a validated range of 1-100 ng ml-1. Due to sensitivity requirements established during the first clinical study, the analysis was transferred to a VG 70S high resolution mass spectrometer and revalidated. The quantitation range of the current assay is 0.1-250 ng ml-1. To date, the method has been used to analyse several thousand blood samples from remifentanil clinical studies.