Our aim was to confirm in our environment what has been observed and described by other writers about the importance of achieving a "critical body weight'' and an adequate "fat percentage'' -on the basis of the calculation of total body water- for the initiation and development of pubertal events. This study included 92 girls, healthy, well nourished, belonging to upper middle class from a high school of The National University of Cordoba. The longitudinal method of control was used every 6 months and at the precise moment of menarche. Out of 20 antropometrical variables observed height, weight and height, TBW as percentage of body weight, lean body and fat weight, fat percentage and skin folds ppercentiles for each girl at menarche. A regression between fat percentage and skin folds was done. Percentiles 5 to 95 of fat percentage in relation to body water percentage were estimated. At menarche the average for the different variables are: Heigth 155.6 cm +/- 0.469; Weight 45.8 Kg +/- 0,5; TBW 25.216 lit. +/- 0.318; lean body weigth 35.02 Kg (S.D.2.98); fat weigth 10.86 Kg (S. D. 3.17). The addition of skin folds was correlated fat percentage, thus, an equation was obtained for the average calculation of such percentage %F= 12.16 + (0.313 x fold addition). The minium percentage for the onset of menstrual cycles is 17.3% and corresponds to percentile 10. However, there is a 5% of girls who start to menstruate with a 15.5% of fat and none of them is below that value. The reasons mentioned above suggest that is necessary to obtain a "critical body weigth'' as well as a "fat percentage'' minimum for the onset and maintenance of menstrual cycles, among our girls, similar o what has been obtained by doctor Frisch.