Sufentanil and its two primary metabolites, N-(4-[methoxymethyl]-4-piperidinyl)-N-phenyl propanamide (MPPP), and N-(4[hydroxymethyl]-1-[2-thienylethyl]-4-piperidinyl)-N-phenyl propanamide (desmethyl sufentanil), were detected by capillary gas chromatography with a nitrogen-phosphorous detector. The detection limit for sufentanil and its metabolites is 30–50 pg/ml with minimal interfering substances in the chromatograms. This method allowed for the detection of serum sufentanil in the terminal elimination phase of sufentanil in a patient receiving 1.5 μg/kg and will allow for studies to determine the pharmacokinetics and metabolism of sufentanil in a wide variety of patient groups now receiving this agent.