SUMMARY: A longitudinal study was made of the urinary excretion of a number of C19 and C21 steroids in 11 healthy boys and 9 healthy girls aged 8–12 yr. Urine collections were made every 6 months, over periods ranging from 1 to 2½ yr. in different children. The excretions of most substances showed surprisingly regular increments in individual children. Differences between individual children were marked and often consistent; some children excreted high amounts of one substance and low amounts of another, and other childen the reverse. The variation between individuals in the excretion of substances such as androsterone and aetiocholanolone was reduced when the results were plotted against skeletal rather than chronological age. DHA was excreted by all subjects, but at a very low level before the skeletal age of 10 yr. The glucuronide: sulphate ratio of the 11-deoxy-17-oxosteroids fell consistently from 8 to 12 yr. whereas the ratio of 5α-: 5β-11-deoxy-17-oxosteroids increased. Boys excreted more C19 steroids than girls with the same skeletal maturity score. This comparison is physiologically more meaningful than comparing boys and girls of the same chronological age.