The pituitary and adrenal activitiesin untreated congenital adrenal hyperplasia (CAH) were studied. Two children had 11β-hydroxylase deficiency: subject 1, 8½ yr, and subject 2, 6½ yr. The daily 11-desoxycortisol and cortisol secretionrates were 47 and 18 mg respectively, for subject 1 and 32 and 10 mg respectively for subject 2. The plasma half-life, distribution volume and MCR of 11-desoxycortisol in subject 2 were 22 min., 38.5 l and 1750 l/day. The values for cortisol in subject 1 were T½ = 45 min., 26.6 l distribution volume and MCR of 590 l/day. Two children had 21-hydroxylase deficiency: subject 3, 5 10/12 yr, and subject 4, 10 months old. In subject 3 the plasma T½, apparent distribution volume and MCR of cortisol were 51 min., 11.7 1 and 230 l/day. It was calculated that subject 3 produced 35 mg of 21-desoxycortisol and 9 mg of cortisol daily. Urinary steroid metabolites were studied in all 4 subjects; the patterns weretypical for their respective genetic defect. Plasma corticosteroid concentrations in subjects 1 and 3 and in 6 children evaluated for short stature were determined at 20-min intervals for 24 hr. The adrenal hormones were secreted episodically. The secretory patterns of the CAH subjects were similar to those of short stature children and normal boys matched for age and sexual maturity. The pattern of adrenal secretory activity was examined by the amounts of corticosteroid produced at different periods of the day. The distribution of the fraction of daily corticosteroid in these various periods was similar in all groups. Simultaneous meas-urements of plasma ACTH and corticosteroids were carried out between 1100 and 1700 hr at 10-to-20-min intervals in the CAH subject 1 with 11β-hydroxylase deficiency, the 2 CAH children with 21-hydroxylase deficiency and 2 children with precocious pubarche in the same chronological agerange. The ACTH concentrations ranged between 0–15 ng/day, and were comparable to those observed in the 2 children with precocious pubarche and in normal adults. Except for greater synthesis and secretion of adrenal hormones, the pituitary-adrenal interaction in the CAH subjects studied appear to perform in a normal fashion.