Carbamazepine was administered to nine patients with manic-depressive illness at doses ranging from 600 to 1600 mg per day, achieving blood levels between 6 and 11 microgram per milliliter. Compared to medication-free values, GABA levels in the cerebrospinal fluid (CSF) were not significantly altered by an average of 30 days' treatment with carbamazepine. These preliminary data do not support a major effect of carbamazepine on brain GABA as reflected in CSF as a mechanism for its anticonvulsant or psychotropic effects.