AbstractA reaction sequence is described to synthesize p,p′‐dideuteriophenytoin in three steps from commercially available starting materials. The deuterium content at best reached 99.2% as measured by mass spectrometry. For materials with isotopic content up to 95%, excellent agreement was found between the data obtained by mass spectrometry and13C‐nuclear magnetic resonance operated in a NOE‐suppression mode. p,p′‐Dideuteriophenytoin was evaluated comparatively with phenytoin and p‐deuteriophenytoin in the maximal electroshock test. While all three compounds exhibited a high degree of antiepileptic activity, none of them demonstrated significant superiority ov