3-(2,6-Xylyl)-5-methylhydantoin--a metabolite or a metabonate of tocainide in rats.

1. G.1.c.-mass spectrometric analysis of urine extracts from rats dosed with tocainide (I) revealed the presence of cyclic compound identified as 3-(2,6-xylyl)-5-methylhydantoin (IV) derived from tocainide. 2. Evidence indicates that IV is derived from a conjugate of tocainide (tocainide carbonyl O-beta-D-glucuronide) (III) in humans. 3. Results of the present study indicate that in addition to being formed from a conjugate, IV is generated directly from tocainide in vivo in rats. Possible mechanisms for the formation of IV in rats in vivo are discussed.