Phenytoin (PT) and carbamazepine (CBZ) are the two most prescribed anticonvulsants in Finland. Their effect on the cognitive functions of 43 newly diagnosed epileptic patients was examined. The medication was randomly assigned. The patients were tested before the medication was started, and after half a year's therapy. In order to estimate the practice effect in repeated testing a control group of 21 volunteers was similarly tested and retested. Both anticonvulsants, PT in particular, decrease the normal practice effect observed in neuropsychological testing. Compared to the CBZ group, patients with PT became somewhat slower, and their visual memory decreased. Within the PT group the motor slowing was more marked in female patients, and in patients having higher PT serum levels. In PT and CBZ groups there was an equal decrease in negative mood i.e. tension, depression, bewilderment and irritability.