An assay was developed for determining mefloquine (quinolinemethanol) and pyridinemethanol derivative concentrations in whole blood. The method involved ion-pair extraction or usual solvent extraction for drug recovery from whole blood followed by trimethylsilylation. The silylated compounds were then submitted to GLC with electron-capture or flame-ionization detection. Mass spectrometry combined with GLC of the trimethylsilyl derivatives indicated that substitution of one trimethylsilyl group had occurred on the hydroxyl group. A phenyl methyl silicone column with temperature programming separated the drugs from normal blood extracts. The determination limit was 10 ng/ml of whole blood when an electron-capture detector was used with ion-pair extraction. Quantitation was achieved by using one antimalarial as an internal standard for the assay of the other. The utility of the present method was demonstrated by following the whole blood level time course after a single oral 250-mg tablet in beagle dogs.