[7-3HA1Androstenedione and [4-14C]estrone 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 androstene tissue. In eight men the mean +/- SE value for the fractional 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 less than 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.