A two-period, crossover bioavailability study was conducted to evaluate the relative and absolute bioavailability of immediate (IR) and extended-release (XR) lamotrigine formulations under steady-state conditions in elderly patients with epilepsy. On treatment days, each subject's morning dose (IR or XR lamotrigine) was replaced with an intravenous 50 mg dose of stable-labeled lamotrigine. Lamotrigine concentrations were measured at 13 time points between 0 and 96 hours. XR and IR lamotrigine formulations were similar with respect to steady-state area-under-the-concentration-time curve (AUC0-24 , ss ), average concentration (Cavgss ) and trough concentration (Cτss ). A 33% lower fluctuation in concentrations with XR was observed relative to IR lamotrigine. The time to peak concentration (Tmaxss ) was delayed for XR lamotrigine (3.0 hours vs. 1.3 hours) with lower peak concentration (15% lower). The absolute bioavailability for IR and XR formulations were 73% and 92%, respectively. The formulations were bioequivalent with respect to AUC0-24 h, ss , Cτss , and Cavgss indicating that it may be possible to switch directly from IR to XR lamotrigine without changes in the total daily dose.