A single oral dose of lamotrigine was administered to seven volunteers with Gilbert's Syndrome (unconjugated hyperbilirubinaemia). Plasma samples were assayed by high performance liquid chromatography (h.p.l.c.) and pharmacokinetic parameters were compared with those of a group of nine normal volunteers. In the subjects with Gilbert's Syndrome mean oral clearance (CLpo) was 32% lower (P less than 0.01) and the plasma elimination half-life (t1/2) was 37% lower (P less than 0.02) than in the normal controls. The amount of unchanged lamotrigine excreted in the urine was 30% greater in the Gilbert's subjects (P less than 0.01) although this only amounted to 9.5% of the administered dose. Subjects with Gilbert's Syndrome have some impairment of lamotrigine elimination but this is unlikely to be clinically important.