[7-³H]Androstenedione and [4-¹⁴C]es-trone or [7-³H]testosterone and [¹⁴C]estradiol were infused at constant rates into brachial arm veins of 15 normal men. During the infusions blood samples were obtained from the brachial artery, a deep vein draining primarily muscle, and a superficial vein draining primarily adipose tissue of the arm contralateral to the infusion. In seven men the mean ± SE value for the fractional conversion of androstenedione to estrone was 0.013 ± 0.004 for muscle and 0.014 ± 0.002 for adipose tissue. In eight men the mean SEvalue for the frac-tional conversion of testosterone to estradiol was 0.0007± 0.0001 for muscle and 0.0012 ± 0.0002 for adipose tissue. Both of these values were significantly (P < 0.01) less than the respective values of androstenedione aromatization to estrone.

If constancy of tissue aromatization throughout the body is assumed, the muscle accounts for 25-30% and adipose tissue for 10-15% of the total extragonadal aromatization of androgens to estrogens. (J Clin En-docrinol Metab46: 146, 1978)