Ten patients, suffering from affective disorders, were treated with carbamazepine for polyuria and polydipsia associated with long-term lithium therapy. Oral carbamazepine (300--600 mg daily for six weeks) was observed to have no beneficial effect in alleviating these symptoms when compared with placebo tablets in a double blind crossover study. Plasma and urinary osmolality were observed to be within normal range in these patients and there was no antidiuretic response following subcutaneous Pitressin injection. There was 50% drop-out due to severe side-effects like ataxia, dizziness, restlessness and confusional states. It appears that lithium exacerbates carbamazepine induced CNS side-effects or vice versa, the mechanism of which is not very clear. It may be due to their mutual effect on sodium metabolism or on nervous conduction velocity. Hence, simultaneous administration of these two drugs should preferably be avoided.