Using constant infusions of [3H]testosterone and [14C]oestradiol or [3H]androstenedione and [14C]oestrone the dynamics of androgen and oestrogen metabolism and production in patients with hyperthyroidism were measured. The metabolic clearance rates of testosterone and oestradiol were decreased but those of androstenedione and oestrone were within the normal range. The conversion ratios of testosterone to androstenedione and of testosterone to dihydrotestosterone (DHT) were decreased whereas those of androstenedione to testosterone and androstenedione to DHT were increased. These changes could be explained by increased serum levels of sex hormone binding globulin which binds testosterone and DHT but not androstenedione. The fraction of androstenedione infused into and measured as oestrone in the blood was normal in seven out of nine subjects and the fraction of testosterone infused and measured as oestradiol was normal in all nine subjects. The production rates of testosterone and oestradiol were in the normal range but the production rates of androstenedione and oestrone were raised in half the subjects.