Androgen action in sexual tissues, especially skin and the prostate, is expressed by dihydrotestosterone (DHT) acting at the nuclear level. Dihydrotestosterone in the circulation and target tissues is almost solely derived from the peripheral conversion of secreted testosterone (T) in men and androstenedione in women. The general pathway is testosterone----DHT in equilibrium with androstanediol (3 alpha diol). However, a number of studies suggest that blood DHT or 3 alpha diol are not reliable indicators of peripheral DHT formation. This is particularly suggested by discrepancies in the specific activity of DHT in blood and urine following infusion of labeled DHT, suggesting that total body DHT formation is not reflected by blood levels. Thus, DHT should be thought of as a paracrine hormone formed and acting primarily in target tissues. 3 alpha androstanediol glucuronide (3 alpha diol G) is a major metabolite of DHT. An important site of its formation is the skin. Levels in blood and urine are increased in hirsutism and acne, and blood levels closely parallel pubertal development. 3 alpha diol G levels are especially increased in adrenal disorders of androgenicity such as andrenogenital syndrome; it is also a good marker of response to therapy. Levels are reduced in various forms of male pseudohermaphroditism. 3 alpha androstanediol glucuronide appears to be the best marker available of DHT formation in target tissues such as skin.