The gap between the accerelated physical growth and the stayed or relatively delayed mental development has become common in many countries. In this sense, the study on sexual maturation has to cover not only the physical but also the mental aspects. Age at menarche, the knowledge of sexual phenomena, and the behavioural pattern at counselling of sexual problems were the content of questionnaire. The subjects were girl students at Eiwa Junior and Senior High School in Yamanashi Prefecture. The results are summarized as follows: (1) Age at menarche has become younger annually. (2) Psychological reaction toward menarche differed dependent upon the age at it. (3) After menarche, students became more conscious to sexual phenomena, and tended to select persons other than parent as the counseller. (4) The amount of knowledge on sexual phenomena increased with the chronological age, but the understanding of underlying mechanism did not show a simply increasing pattern. (5) The age at menarche and the feeling invited by the menarche seemed to influence the increase of knowledge and the deepening of understanding. (6) By the content analysis of the answer to questionnaire, the development in intelligence was supposed to relate closely to the advancement of knowledge on sexual matters.
