The extractive alkylation of 11 sulphonamide diuretics has been evaluated using tetrabutylammonium, tetrapentylammonium or tetrahexylammonium as counter ion at different pH values and methyl iodide in methylene chloride as the organic phase. The sulphonamides are methylated within 20 min with tetrahexylammonium as counter ion in 0.2 M sodium hydroxide at 50 degrees. The derivatives have been identified by mass spectral and nuclear magnetic resonance analysis. Relative retentions of the derivatives are given using 1% SE-30 as the stationary phase. A contaminant, dimethylsulphuric acid, occurs in methyl iodide and seriously disturbs the gas chromatographic analysis. The application of the extrative alkylation to biological samples is demonstrated by the direct analysis of acetazolamide in serum. 0.1 M tetrapentylammonium in 0.5 M sodium hydroxide is suitable as the aqueous phase with 1.6 M methyl iodide as alkylating reagent in methylene chloride. The trimethyl derivative of acetazolamide formed has been determined by electron-capture gas chromatography down to 0.5 microgram/ml in a 0.1-ml serum sample. The relative standard deviation at the 10 microgram/ml level is 6.6% (n = 10).