Paraxanthine-d6 is intended for use as an internal standard for the quantification of paraxanthine by GC- or LC-MS. Paraxanthine is an active metabolite of caffeine. It is formed via N3-demethylation of caffeine by the cytochrome P450 (CYP) isoform CYP1A2. Paraxanthine is an adenosine A1 and A2 receptor antagonist. In vivo, paraxanthine increases striatal cGMP and extracellular striatal dopamine levels and locomotor activity, as well as inhibits motor depression induced by the adenosine A1 agonist CPA or the adenosine A2 receptor agonist CGS 21680 in rats not habituated to caffeine. It also promotes wakefulness and increases locomotor activity and core temperature in narcoleptic transgenic mice without increasing behavioral anxiety.