The urinary excretion of free and peptide bound hydroxyproline as well as creatinine and total nitrogen was measured in 198 infants, children, adolescents as well as 11 women aged 20 years. From the data the ratio of urinary hydroxyproline to body weight, creatinine and total nitrogen excretion was calculated from the 1st month of life to the age of 20 years. It is shown that with regard to the above mentioned parameters the largest amount of hydroxyproline is excreted with the urine in the 1st year of life. This also holds true when only the bound hydroxyproline is taken into account. After a significant fall, the relationship between hych'oxyproline excretion and body weight, urinary creatinine and nitrogen remains practically constant or decreases minimally between the age of 2 and 20 years. The increased rate of growth during puberty manifests itself in a rapid increase of the absolute value of the excreted hydroxyproline from the age of 9 years onwards. This increase parallels the gain in weight and height (in girls earlier than in boys). At the same time creatinine and nitrogen excretion rises. The absolute hydroxyproline values are considerably lower in girls between the ages of 14 to 17 years than those in boys of the same age,
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