In 2006, brownish-pink clandestine tablets with an “S” stamp mark were seized in several prefectures in Japan. These were reportedly sold as “Ecstasy (MDMA tablet)”, but routine analysis indicated no traces of any controlled substances or common recreational drugs. However, additional high resolution mass spectrometry and nuclear magnetic resonance spectroscopy confirmed the unusual component N -(2,4,6-trimethylphenyl)phthalimide (TMPPI). Because this compound is considered to be a derivative of Thalidomide, potential sedative and anticarcinogenic effects were expected from its structure.   To investigate its metabolism and toxicity, administration experiments were conducted on rats. As a result, N -( o -carboxybenzoyl)-2,4,6-trimethylaniline was identified as a specific metabolite in the rats' urine. Furthermore, no symptoms were noticed with a single dose of 350 mg/kg TMPPI. Thus, TMPPI was found to have a low acute toxicity.