The development of a gas-liquid chromatographic method9 for the measurement of propoxyphene (Darvon) in plasma and extension of this technique to norpropoxyphene6, 8 has led to the examination of the kinetics of the drug and its primary metabolite.2, 5, 6, 8, 10 Calculations indicated that a considerable “first pass effect”3 influences drug plasma levels. Low assay sensitivity has prevented plasma level measurement for the extended periods of time necessary to establish terminal plasma concentrations.7