Total body water was determined on 59 boys and 59 girls and on 40 boys and 45 girls attending junior and senior high schools, respectively. The volunteers were studied for three years. They were given D2O (99.8%) orally, and urine samples were used for determination of the ratio of heavy hydrogen to normal hydrogen by mass spectrometry. Total body water was calculated. The height of boys aged 12 to 13 years increased from 159.1 cm to 177.7 cm at age 17 to 18 years. The girls' height changed from 158.5 cm to 165.7 cm for the same time period. Similarly, the weight of boys increased concomitantly from 49.44 kg to 66.1 kg, whereas the girls' weight increased from 49.0 kg to 58.3 kg. Total body water of boys aged 12 to 13 years and expressed in absolute values increased from 31.9 L to 43.3 L at age 17 to 18. Similar changes of smaller magnitude occurred in girls, where total body water increased from 29.0 L to 31.9 L for the same period of time. In relative terms, total body water of boys increased from 62.1% to 65.9%, reached at the age of 15 to 16 years, and then a slight decrease to 64.2% was found in boys 17 to 18 years old. The girls showed continuous decrease in relative values from 60.3% to 53.6% during the same period of time.(ABSTRACT TRUNCATED AT 250 WORDS)