A method for the determination of cytosine arabinoside in the plasma of leukemic patients being treated with this drug is described using either gas--liquid chromatography with a nitrogen-sensitive flame ionization detector or gas chromatography--mass spectrometry (GC-MS). To increase volatility, a double derivative of cytosine arabinoside was used, prepared by acetylation and subsequent methylation. Cytidine was used as internal standard for the GC procedure. GC--MS was performed with either cytidine as internal standard and detection by single-ion monitoring or by the use of [2H3] acetate-methyl derivative of cytosine arabinoside as internal standard and subsequent multiple-ion monitoring. Attempted extraction of cytosine arabinoside from plasma with various organic solvents was unsuccessful, but protein precipitation with ethanol or trichloroacetic acid followed by washing of the aqueous residue with organic solvents to remove as many of the interfering substances as possible gave satisfactory results. The minimum detectable quantity of pure cytosine arabinoside was similar for both techniques (approximately 500 pg). However, with GC using a nitrogen-sensitive detector, the lower limit of detection from plasma was found to be approximately 40--70 ng per ml plasma whilst GC--MS showed greater analytical selectivity with a detection limit in some cases as low as 1 ng per ml plasma.