The effect of straight release carbamazepine monotherapy was studied in 12 well-controlled epileptic patients using adaptive tracking, smooth pursuit and saccadic eye movements, body sway, Digit Symbol Substitution Test (DSST) and Visual Analogue Scales. Patients were matched to healthy controls for age and gender. After patients had received their usual morning dose of carbamazepine, patient-control pairs were studied for 7 h. Compared to controls, the average DSST scores of patients were significant lower. No relationships were shown between DSST performance and plasma concentrations of carbamazepine and carbamazepine-epoxide. No significant differences were found for any of the other effect parameters. Variations in plasma concentrations were limited, contributing to the absence of systematic fluctuations in test results. Of the used tests, DSST is most clearly related to cognitive function. It is concluded that the difference in DSST performance appears to reflect a long-term small neurocognitive difference between subjects with and without epilepsy.