We studied the effect of electrical stimulation of centromedian thalamic nuclei (ESCM) on seizure control and paroxysmal EEG activity in 23 patients. We report the effect of chronic ESCM on psychological performance and background EEG activity of patients with various intractable seizure patterns. In each patient, a simple specifically designed neuropsychological scales (one for adults and one for children) was administered at the end of the baseline (BL), ESCM, and poststimulation (Post) periods; and 14 consecutive EEG recordings during these periods were performed to determine the degree of neuropsychological improvement and the temporal course of EEG changes. A significant increase in psychological scores and the number of background EEG waves per 10 s was noted in groups A (generalized tonic-clonic seizures, GTC), C (complex partial seizures, CPS), and D (generalized tonic seizures) and the total group of patients from BL to ESCM and from BL to Post periods. Group B patients showed a substantial increase (partial motor seizures) during the same periods. Improvement on psychological performance correlated better with age and baseline degree of deterioration than with the nature of the particular psychological improvement in any given subtest. The improvement in EEG background rhythm was most noticeable at the end of ESCM and at the beginning of the Post periods. Complete normalization of neuropsychologic scores and EEG rhythms was rare, but improvement was significant for both. Psychological scores increased from BL 14 +/- 2 to ESCM 21 +/- 2 and Post 23 +/- 2 (normal expected 26), and EEG background rhythm increased from BL 42 +/- 2 to ESCM 62 +/- 2 and Post 54 +/- 2 EEG waves/10 s. (normal expected > 80).