1. The metabolic profile of D-23129, a new anticonvulsant agent, was studied in vitro using human liver microsomes and fresh liver slices. 2. Oxidative metabolism appeared to be minimal with D-23129. The percent mean total radioactivity not associated with the parent compound recovered from oxidative metabolism studies from three individual liver donors was 0.7% +/- 0.6 SD and was not significantly different from [14C]-D-23129 incubated with heat inactivated microsomes, mean = 0.5% +/- 0.4 SD. 3. Phase II conjugation dominated the metabolism of D-23129 producing two distinct N-glucuronides as the primary metabolites. These metabolites were identified by electrospray ionization LC/MS. 4. The apparent Km for one of the glucuronide metabolites was determined in human liver microsome preparations from two individual liver donors to be 131 and 264 microM respectively, Vmax determined for the same microsomal preparations yielded 48.9 and 59.9 pmol/min/mg protein.