The aim of this study was to determine whether methamphetamine (MA) is metabolized to 4-hydroxy-3-methoxymethamphetamine (HMMA), which is known as the main metabolite of 3,4-methylenedioxymethamphetamine (MDMA). After MA was intravenously administered to rats, the plasma, urine, and bile were collected periodically. HMMA together with MA and its main metabolites, amphetamine and 4-hydroxymethamphetamine, were detected in the rat plasma, urine, and bile by gas chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry. In addition, HMMA was produced when MA was incubated with human liver microsomes. HMMA may be produced as a metabolite of MA when humans have consumed MA, although the amount of HMMA would be small compared with that of MA, amphetamine, or 4-hydroxymethamphetamine. The results of the present study will be helpful in determining the type of drug used.