Combined anticonvulsant effects of phenytoin and phenobarbital in mice and rabbits were studied using the isobolographic method which was based on the plasma concentrations of these drugs. Anticonvulsant activity of test drugs alone or in combination was determined by the method of maximal electroshock seizure test. Response of each animal to drug action was recorded as abolition of the tonic hindlimb extensor component of maximal electroshock seizure. It was demonstrated that the minimal effective plasma concentrations of phenytoin and phenobarbital alone for protection against maximal electroshock seizure was 5.9 +/- 0.2 and 18.1 +/- 0.4 microgram/ml (+/- SE) in mice and 14.7 +/- 0.5 and 17.5 +/- 0.6 microgram/ml (+/-SE) in rabbits, respectively. Isobolograms for mice and rabbits were constructed by identifying and plotting each responder and nonresponder for its corresponding plasma phenytoin and phenobarbital concentrations when these drugs were given in combination. The isoboles in mice and rabbits were concave bowing toward the origin, indicated synergism (potentiation). The apparent synergism between phenytoin and phenobarbital cannot be explained by changes in the amount of these drugs entering into the brain from the blood, since the relationship between brain and plasma concentrations of either drug was not altered by the presence of the other. These results, therefore, suggest that the synergistic action between these two anticonvulsants occurs within the central nervous system; this provides experimental support for the combined use of phenytoin and phenobarbital in the clinical treatment of epilepsy.