A method for qualitative and quantitative detection of nanogram amounts of O6-monoacetylmorphine (MAM) in urine samples of heroin abusers is presented. The detection of MAM is based on the alkaline solid phase extraction of the urine samples using an octadecyl column and transformation into the pentafluoropropionyl (PFP) derivatives. PFP-MAM is separated by capillary GC and identified mass spectrometrically by selected ion monitoring (SIM). The determination of the MAM levels was carried out by quantitative GC/SIM using nalorphine as the internal standard. A positive identification of MAM allows one to distinguish between a previous intake of heroin (or theoretically MAM) on the one hand, and an ingestion of morphine, codeine, opium, or poppy seed on the other.