Carbamazepine and valproate are two clinically used anticonvulsants which are also effective in the treatment of manic-depressive illness. Although the biochemical profiles of these drugs are markedly different, some mechanisms in common may be implied by their partially overlapping spectrum of therapeutic efficacy in seizure and affective disorders. Further evaluation of common biological targets of these agents was attempted by determining whether cross-tolerance would occur to the anticonvulsant effects of carbamazepine and valproate on amygdala-kindled seizures. It had previously been shown that tolerance to carbamazepine's anticonvulsant effects on amygdala-kindled seizures occurs only with contingent drug administration, i.e., it occurs only when the drug is injected before the kindling stimulation, and not when the drug is given after the seizure. In the current studies, rats that were made tolerant to carbamazepine showed cross-tolerance to valproate. Kindled rats given carbamazepine after each seizure stimulation (i.e., non-tolerant controls) did not show tolerance to valproate's anticonvulsant effects, indicating that the cross-tolerance between carbamazepine and valproate was also contingent. The clinical implications and potential common biochemical target mechanisms of the cross-tolerance between carbamazepine and valproate deserve further investigation.