A sensitive and selective gas chromatography method using flame ionization detection was developed for the determination of diethylcarbamazine (DEC) in human plasma. DEC and the internal standard, 1-diethylcarbamyl-4-ethyl piperazine HCl (E-DEC), were extracted from human plasma after loading onto a conditioned C(18) solid phase extraction cartridge, rinsed with water and eluted with methanol. After evaporation under a stream of nitrogen and reconstitution in methanol, 3 microl were injected onto the GC system. Separation was achieved on a A Heliflex(R) AT-35 capillary column (length 30 m, internal diameter 0.32 mm). Gas flow rates were: hydrogen, 35 ml/min; carrier gas (helium), 1.5 ml/min, make-up gas (helium), 25 ml/min; and air 420 ml/min. The retention times of DEC and internal standard were approximately 5.5 and 7.28 min, respectively. The GC run time was 22 min. The assay was linear in concentration range 100-2000 ng/ml for DEC in human plasma. The analysis of quality control samples for DEC (120, 1000, 2000 ng/ml) demonstrated excellent precision with coefficients of variation of 4.5,1.3, and 1.6%, respectively (n=6). The method was accurate with all intra-day (n=6) and inter-day (n=12) mean concentrations within 4.3% from nominal at all quality control sample concentrations. DEC was found to be stable after 3 freeze-thaw cycles, and with storage at -20 degrees C for 12 weeks. The method is currently being used for pharmacokinetic studies of DEC in healthy volunteers.