To measure the rate of aromatization that occurs in splanchnic tissue, four men with normal liver function were infused through an arm vein with [3H]androstenedione/[14C] estrone and four men were infused with [3H]testosterone/[14C] estradiol before indicated cardiac procedures. Catheters were placed under fluoroscopic control in the hepatic vein and descending aorta. Simultaneous blood samples were then obtained from the aorta and the hepatic vein after 90 and 120 min of infusion. The samples were analyzed for radioactivity after multiple chromatographic purification steps. The MCRs, overall aromatization (fraction of androgen infused measured as estrogen in arterial blood), splanchnic extractions, and splanchnic aromatization (fraction of androgen entering the splanchnic tissue which leaves as estrogen) were determined. The mean values for MCRs of androgens and estrogens and overall aromatization for androstenedione and testosterone were similar to those reported previously. The splanchnic extraction values of androstenedione, testosterone, estrone, and estradiol were 0.57 +/- 0.10 (+/- SE), 0.39 +/- 0.12, 0.60 +/- 0.09, and 0.64 +/- 0.18, respectively. The mean splanchnic aromatization of androstenedione to estrone was 0.00086 +/- 0.00072, and the mean splanchnic aromatization of testosterone to estradiol was 0.00059 +/- 0.00020. If one assumes that the splanchnic bed receives 20% of the cardiac output, then the splanchnic bed is responsible for less than 4% of the overall peripheral aromatization of androstenedione or testosterone.