This study investigated correlations between the EEG and 10 different chemical indices in 23 uremic patients, tested over long periods of time (up to 18 months). An abnormal EEG was recorded at least once in 70% of these patients, but among the 362 tracings from all patients, 36% were considered abnormal. Slow wave abnormalities, mainly mild in degree and frontal in location, were found in 97% of abnormal records and epileptiform patterns in the form of bilateral spike and wave complexes were seen in 14%. Patients with these EEG abnormalities had a significantly slower frequency of background rhythm than patients with normal EEGs. Compared to those with normal records, patients with abnormal EEGs had a significantly higher BUN and Cl and lower Ca, as determined by standard (absolute) mean values. With relative changes, based on the percentage change from the previous value, the BUN, Ca, Cl, K, PO4 and CO2 showed significant relationships with a deteriorating EEG and the Ca, Cl, K and CO2 with an improving record. A deteriorating EEG was usually associated with an increasing BUN, Cr, K, Cl and Na and a decreasing Ca, PO4, Hct, Alb and CO2. Significant absolute changes in the chemical indices were more often seen than relative ones, especially with a deteriorating EEG. With either absolute or relatives values, the BUN showed the closest relationship with EEG deterioration and the Cr, Cl and Hct with EEG improvement. Changes were often seen in a combination of chemical indices, most often involving the BUN and to a lesser extent the Cr. With combined changes the one with the greatest change was especially the BUN and PO4. Variations in the amount of epileptiform activity were most closely related to changes in the BUN. The main conclusions of this study are that the BUN correlated best with the EEG among the 10 different serum values using various indices, especially standard (absolute) values with a deteriorating recording. The second highest number of correlations were found with the Cl, especially absolute values with an improving EEG. The Ca and K showed the best correlation with relative changes. Thus, correlations of various chemical indices with electrographic changes in uremic patients depend upon whether absolute or relative values are used and whether EEGs are deteriorating or improving.