Methamphetamine is a popular drug of abuse, which is readily synthesized in clandestine laboratories. Illicit synthesis results in the formation of various contaminants. Few impurities have been studied in vivo, and their metabolic fate is unknown. One such impurity is alpha-benzyl-N-methylphenethylamine (BNMPA). The detection of BNMPA or its metabolites in urine samples may provide a marker of use of illicitly synthesized methamphetamine. Benzphetamine is structurally similar to BNMPA. Based on metabolic studies of benzphetamine, we predicted the four major metabolites of BNMPA to be the N-demethyl compound, diphenyl-2-propanone (DP2P), p-hydroxy-N-demethyl BNMPA, and p-hydroxy-BNMPA. One male volunteer ingested 5 mg BNMPA. Seventeen urine specimens were collected over 50 h post ingestion. These specimens were analyzed for BNMPA and its four predicted major metabolites by gas chromatography-mass spectrometry following beta-glucuronidase hydrolysis or acid hydrolysis, liquid-liquid extraction, and derivatization with heptafluorobutyric anhydride. Specimens were also analyzed without hydrolysis to determine the abundance of nonconjugated ("free") metabolites. Only trace amounts of BNMPA and its N-demethyl metabolites were detected, and maximum excretion was from 2 to 4 h post ingestion. In the nonhydrolyzed samples, the phenyl-OH metabolites were also present in only trace amounts. Maximum excretion of DP2P was at 2 h. Following either hydrolysis procedure, phenyl-OH-BNMPA and phenyl-OH-N-demethyl BNMPA were the major metabolites detected. Maximum excretion of these two metabolites occurred at 4 h. With the exception of the parent compound and the N-demethyl metabolite, excretion of metabolites was greater than the limit of detection of this procedure (2.5 ng/mL) up to 21 h post ingestion. Metabolites were detectable in sufficient quantities to serve as an adequate marker of illicit methamphetamine consumption within the preceding 24 h.