The effect of lithium administered for six months on simulated car driving and other psychological tests was studied in patients with Menière's disease. The dose of lithium was adjusted every two weeks to maintain the serum level between 0.7 and 1.0 mmol/1. The trial was double-blind and cross-over in type, the effect of lithium being compared with a placebo. The subjects were within the normal range of Beck's depression scale and Marke-Nyman's temperament scale. Lithium was found neither to influence the simulated driving nor to affect the scores in the two rating scales. The only specific complaints observed during lithium treatment were tremor and increased thirst.