The plasma concentration, production rate, and conversion ratio of androstenedione and testosterone were studied in seven children with congenital adrenal hyperplasia (CAH) of the 21-hydroxylase type. Plasma androstenedione and testosterone measured by double isotope derivative assay and estimated blood production rates were manyfold increased in the untreated state, markedly suppressed with glucocorticoid, and increased after the administration of ACTH. The metabolic clearance rate when corrected for body size and the conversion ratio of androstenedione to testosterone were similar to previously determined values in normal adults. Consideration of the androgen concentrations and conversion ratios indicates that in children with CAH, 76% of the plasma testosterone in prepubertal females and 36% in males are derived from peripheral conversion of blood androstenedione. The calculated amount of testosterone unaccounted for by peripheral conversion is similar to normal prepubertal values. This approach indicates that virilization in these children results from increased levels of testosterone but that the major source in CAH of this potent androgen is androstenedione secreted by the adrenal cortex.