A new analytical method for the determination of non-steroidal acidic anti-inflammatory drugs in water samples is described. These compounds are used as anti-inflammatory, antipyretic and analgesic drugs in human health care and in veterinary applications. Analytical procedure involves in situ derivatization of analytes to their methyl esters with dimethyl sulphate, headspace sampling using solid-phase microextraction (SPME), and gas chromatography-mass spectrometry (GC-MS). The effects of pH, ionic strength, extraction time, SPME fibre, extraction temperature as well as derivatization conditions were studied. Methyl esters were extracted with a fused-silica fibre coated with 100 microm polydimethylsiloxane. Response was linearly dependent on concentration in the range 0.1-10.0 ng mL(-1). Detection limits were achieved at the level of 0.3-2.9 ng L(-1). Derivatization-SPME/GC-MS analysis yielded good precision (R.S.D. between 7.9 and 17.2%). The method was validated by analysis of spiked matrix samples.