Plasma androstanediol-glucuronide (ADG) is considered by many authors to be a highly reliable parameter of peripheral androgenicity. Recently, several authors have questioned the reliability of the ADG levels as a parameter of androgenicity. Our data obtained by continuous infusion experiments showed that in women the adrenal steroids, dehydroepiandrosterone sulfate, androstenedione and dehydroepiandrosterone are the major precursors of plasma ADG, accounting for almost the totality of circulating ADG. As expected, in view of its precursors, ADG levels decrease significantly with age. Dexamethasone causes a significant decrease of these levels, whereas in women with Addison's disease the levels are only 20% of normal levels; ovariectomy hardly influences ADG levels. Our data show that in women with moderate hirsutism, plasma ADG levels are no more often increased than the other androgens. In virilizing syndromes ADG levels are higher than expected from precursor levels, suggesting an increased 5 alpha-reductase activity. In hyperthyroidism as well as in euthyroid women with isolated suppressed thyroid stimulating hormone, ADG levels are increased without any sign of virilism. In men, ADG levels have testosterone as a major precursor, but the adrenals contribute to +/- 30% of ADG levels. After transdermal dihydrotestosterone gel, free androstanediol levels increased by a factor of 40, but ADG levels were only increased by a factor of 4, suggesting that the skin is not very effective in conjugating androstanediol. It is concluded that ADG levels in women reflect essentially adrenal precursor levels as well as 5 alpha-reductase activity in peripheral tissues inclusive of the liver.