The urinary excretion patterns of 6-acetylmorphine (6-AM), free morphine, and total morphine were determined by GC/MS assay for six human subjects who received single doses of 3.0 and 6.0 mg of heroin hydrochloride. Clinical specimens were collected and combined with standardized drug urines into a 400 specimen/standard set. The urines were coded, randomized, and analyzed under blind conditions. The GC/MS assay had a limit of sensitivity of 0.81 ng/mL for 6-AM and displayed a linear response across a concentration range of 1-100 ng/mL. Following heroin administration, 6-AM was excreted rapidly with an average half-life of 0.6 h. This resulted in a very short detection time for 6-AM with a range of 2-8 h at the most sensitive cutoff limit. This short detection time limits the usefulness of 6-AM as a marker for identification of heroin abusers to a period immediately after drug use. In contrast, free morphine and total morphine were detectable up to approximately 24 h after heroin administration. The average half-life for free morphine was 3.6 h and for total morphine was 7.9 h. After morphine and codeine administration, no 6-AM was detected by GC/MS above the 0.81-ng/mL detection limit of the assay. It is concluded that the presence of 6-AM in urine can be interpreted with confidence to mean that heroin, or 6-AM, was administered within 24 h of specimen collection and that the presence of 6-AM in urine is not caused by morphine or codeine administration.