The urinary excretion of steroids was studied in 8 parents of children with congenital adrenal hyperplasia due to 21-hydroxylase deficiency of the simple virilizing and of the salt-losing type. Eight parents of normal children served as controls. 24-hour urines before and after the injection of 40 IU of ACTH were fractionated using gas liquid chromatography on glass capillary columns. Before stimulation no excretion of pregnanetriolone was detected in heterozygous and in normal parents. Following ACTH only heterozygotes showed an excretion of pregnanetriolone in the urine. This averaged 289 mug per 24 h. Employing gas liquid chromatography on glass capillary columns heterozygous carriers of congenital adrenal hyperplasia due to 21-hydroxylase deficiency may reliably be detected by their increased urinary excretion of pregnanetriolone following ACTH.