Carbamazepine and valproic acid are anticonvulsants with mood-stabilizing properties. Basic and clinical research suggest that these medications used together may have synergistic anticonvulsant effects. Psychotropic synergy of the combination has yet to be explored systematically. We present the case of a patient with rapid-cycling bipolar disorder studied under double-blind conditions whose hypomanias and depressions were refractory to either carbamazepine or valproic acid alone, but responded dramatically to the combination. The superior response to the combination appeared to be due to pharmacodynamic rather than pharmacokinetic effects. The clinical and theoretical aspects of the use of carbamazepine and valproic acid in combination are discussed.