In order to see the effects of lithium ion on the EEGs, cat brains were perfused with the standard artificial blood for 30min. and then with the blood containing either 0.810mEq Li(+)/1 (Group 1) or 8.095mEq Li(+)/1 (Group 2) for further 30min. Another group of cats (Group 3) were perfused for one hour with artificial blood where sodium ions were completly replaced with equimolar lithium ions. The EEGs and the concentrations of tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid were compared with those of the standard perfusions. In Group 1, the cortical EEGs showed a slight decrease in the amplitude of the background fast rhythm and a slight increase in slow wave throughout the examinations, and the EEG findings were as same as those of the standard perfusions'. In Croup 2, the amplitude of the background activity and sporadic slow waves were increased and the slight slowing of basic rhythm was observed. However, the EEG findings consisted mainly of the fast rhythm throughout the experiments without remakable changes. In Group 3, during the first 10-15min. the EEGs showed the same pattern as the standard perfusions. During the next 20-45min., however, 200μV or higher voltage spike bursts of 10-30 seconds' duration with the afterdischarge appeared repeatedly. Thereafter the EEGs were reduced to low voltage slow waves or flat pattern until the end of experiments. Therefore, when the brains were perfused with lithium ions together with sodium ions, the electrophysiological activity of the brain was in good function more or less similar to the normal state. However, when the sodium ions were replaced with lithium ions, the brain activity could not be maintained. In Group 3, the brain cortex tryptophan concentrations was decreased to the 60-70% of that of the standard perfusions (p<0.005). In all regions examined, i.e. the cortex, hippocampus, caudate nucleus, thalamus, cerebellum and medulla, the concentrations of 5-hydoxytryptamine were as same as those of the standard perfusions. In the hippocampus the concentrations of 5-hydroxyindoleacetic acid were decreased to 60% of the standard value (p<0.05) and tended to be lower in the caudate nucleus and the medulla, but in other regions they were almost as same as the standard values. Therefore, it is concluded that the metabolism of 5-hydroxytryptamine was inhibited when the cat brains were perfused for one hour with the sodium free blood containing lithium in a high concentration.