The five principal types of posterior slow activity are reviwed: the slow alpha variant rhythm; the posterior theta rhythm; the posterior slow waves at 4 c/s; the delta rhythm associated with petit mal; the so called Pi waves. The results with EEG tracings of 760 patients of both sexes with ages between 5 and 24 years are reviewed. Except for the slow alpha variant, at least one of the four other phenomena as observed in 131 patients (80 males end 51 females). The theta rhythm was observed in 49 cases (29 males and 20 females). This higher frequence among the males is not significant but the higher frequence among the younger and the association with a clinical picture of disturbance of behavior with strong agressivity is significant at 0.01. From 10 patients with diagnosis of petit mal, five presented a posterior slow rhythm (3 males and 2 females). Two patients that presented a posterior slow rhythm at 4 c/s had a history of head injury with consecutive behavior disorders with strong agressivity. The Pi waves were observed in 84 cases (55 males and 29 females), with different curve of decrease with the age in the two sexes. Among the females the decrease of frequence with the age was significant, but among the males it only neared the level of signification. The higher frequence of Pi waves among the males was significant in the age group of 15 to 19 and neared the level of signification in the age of 5 to 9. The Pi waves were present in 100% of the patients with a history of faints, but they had no significant association with the clinical pictures of behaviour disorders, paroxistical headache and dyslexia. It was observed a significant association of the incidence of Pi waves with a good organization of the background activity and with weak or absent response to the hiperventilation (at 0.05). The absense of a significant association between the Pi waves and the theta rhythm suggests that these two electrical patterns are related to quite autonomous phenomena.