Eleven diphenylmethane antihistaminic drugs and their analogues were tested for their detection by capillary gas chromatography (GC) with surface ionization detection (SID). The GC-SID response was highest for doxylamine, diphenhydramine and orphenadrine and lowest for terodiline, clemastine and pipethanate. The detection limits for drugs with the highest response were 2-5 pg (ca. 6-20 fmol) on-column (100-250 pg/ml of body fluid). The detection limits with GC-SID were 10-100 times higher than those with GC with nitrogen-phosphorus detection. A detailed procedure for the isolation of the antihistaminics from human whole blood and urine by the use of Sep-Pak C18 cartridges, prior to GC-SID, is also presented. The recoveries of the drugs (50 or 500 pmol), which had been added to 1 ml of body fluids, were > 60%. The baselines remained steady as the column temperature was increased and the background was clean, especially for whole blood extracts.