Anterior cingular activity may be modified after excision or stimulation of hippocampus; taking this as a principle which is based on experimental and anatomical data, the author thinks that the evolution of some mental and nervous diseases may be modified by hippocampal ablation. Ten patients were operated upon: 5 were submitted to a bilateral ablation of hippocampus; 3 to unilateral ablation; 2 to undercutting of gyrus fusiformis and approach to hippocampus without excision. In patients who were submitted to undercutting of fusiform gyrus, electrical and chemical stimulations of hippocampus were performed after cessation of anesthesia. Electrical stimulation with pulsatile current (5 and 10 cycles per second) produced increase in respiratory rate and amplitude; pulse rate was increased in more than 10 per cent; no change was verified in blood pressure. Acetylcholine plus prostigmine injection produced only increase of diastolic blood pressure. Effects of strychnine injection were like those verified by electrical excitation but less marked. Time and number of cases are still insufficient for a definite conclusion about influence of hippocampal ablation on evolution of some neurological and mental disorders. It seems, however, that behavior disorders in children may be beneficially influenced after bilateral excision.