Specific methods have been devised for the simultaneous determination of the guanidino-type antihypertensive agent debrisoquine and its 4-hydroxy metabolite in human urine, involving in situ derivatisation with acetylacetone, extraction of the resulting pyrimidines, and gas chromatography using a flame-ionisation detector or a nitrogen-specific detector. Using the latter, a similar procedure was also developed to measure debrisoquine in human plasma and whole blood following prior extraction at pH 13.5 with chloroform. Concentrations down to 3 ng/ml can be measured accurately using a 1.0-ml sample. The methods were applied to the analysis of samples collected after a single 20-mg oral dose of debrisoquine hemisulphate.