Carbamazepine and cyheptamide have certain stereochemical features in common with phenytoin; when superimposed, two bulky hydrophobic groups in each permit the approximate apposition of two electron donor atoms. The anticonvulsant activity of each compound was determined in mice using a standard maximal electroshock procedure, and the relative potencies are expressed in terms of the blood and brain concentrations as well as the intraperitoneal dosage. Although cyheptamide was much less potent than carbamazepine or phenytoin on the basis of intraperitoneal dosage, the difference in potency was much less when blood or brain concentrations were compared. These data should be of value for quantitative structure-activity relationship studies.