Tetracyclic antidepressants, mianserin and setiptiline, in human body fluids have been found measurable with high sensitivity by gas chromatography (GC) with surface ionization detection (SID). The compounds in human whole blood or urine samples were extracted by solid-phase extraction using Bond Elut C(18) cartridges. The recoveries of both compounds from the body fluids were above 85%. For quantitation of mianserin, 25 ng of setiptiline was used as internal standard; and for quantitation of setiptiline, vice versa. The calibration curves for spiked whole blood or urine samples were linear in the range of 1.25-50 ng/ml. The detection limit of mianserin and setiptiline was about 1 ng/ml, which is comparable to those obtained by the previous GC-mass spectrometry methods. Our method seems very useful for determination of mianserin and setiptiline in forensic and clinical toxicology, because of high sensitivity, low background impurities and easy handling of the GC instrument.