Twelve subjects received a single oral dose (300 mg) of gabapentin and serial blood and urine samples were collected for drug measurements. Oral phenobarbitone (30-90 mg/day) was then administered to steady-state, and the gabapentin single dose study was repeated on day 42. Gabapentin was administered from days 49 to 52 to achieved steady-state, and further blood and urine samples were collected for drug measurements. Trough plasma phenobarbitone concentrations were monitored at frequent intervals. No statistically significant differences were observed in gabapentin Cmax, tmax, AUC, t1/2 or urinary drug recovery following single doses of gabapentin alone or combined with phenobarbitone. Phenobarbitone did not alter the disposition of gabapentin at steady state. Mean trough steady-state phenobarbitone concentrations were not significantly affected by concomitant gabapentin administration.