The pituitary reserve of GH, ACTH, TSH, LH, and FSH was determined in seven prepubertal birls suffering from congenital adrenal hyperplasia due to 21-hydroxylation defect and under treatment with cortisone acetate. GH and ACTH were studied during the insulin induced hypoglycemia test. The LH, FSH, and TSH reserved were assayed by means of the LH-RH and TRH tests. GH behavior proved to be similar to that found in normal subjects, whereas basal and/or after stimulus ACTH turned out to be higher than the upper limits of the normal range in five out of six girls. The mean basal value and the mean LH peak were not significantly higher than those found in normal prepubertal girls; the mean basal value and the mean FSH peak were lower than the mean of the control group. The difference is significant (P less than 0.05) only between the peak values. The mean basal TSH in the patients is significantly higher (P less than 0.01) than the mean value of the control group. The maximum TSH after TRH is not significantly different from the mean value fo the control group.