Investigation of the influence of pulmonary disease upon local catabolism of cortisol in vivo is limited by the lack of availability of a selective and sensitive assay for simultaneous quantitation of cortisol and cortisone in lung and nasal lavage fluids as well as in plasma. Synthesis of bispentafluorobenzylcarboxymethoxime derivatives of cortisone and cortisol followed by trimethylsilylation yields compounds with almost ideal gas chromatographic properties. The derivatized steroids generate intense M(-.)-181 fragment ions in negative ion chemical ionization mass spectrometry. Monitoring of ions at m/z 829 and m/z 903, respectively, for cortisone and cortisol and corresponding fragment ions generated from multideuterated internal standards provides the analytical basis for assessment of local in vivo conversion of cortisol to cortisone in the respiratory system of human subjects. The assay method displays excellent reproducibility with variations < or = 10%. Detection limits are < or = 1 pg of steroid analyte per injection (S/N > 3). Endogenous levels of cortisone and cortisol, respectively, measured in human samples were as follows: nasal lavage fluid, 47 and 25 pg/ml; bronchoalveolar lavage fluid, 380 and 153 pg/ml; plasma 12.8 and 61.6 ng/ml.