As part of a study on the physiological role of hCG in the human fetus, the hCG concentrations in homogenates of various fetal tissues were measured using a hCG beta subunit RIA. The mean concentrations (picograms of hCG per mg wet tissue +/- SEM; n greater than 10, unless otherwise indicated) found in human fetuses of 12-20 weeks were: ovary, 46.9 +/- 4.3; testis, 8.2 +/- 1.7; kidney, 20.3 +/- 2.8; thymus, 11.5 +/- 1.2; adrenal, 2.6 +/- 0.4; lung, 3.4 +/- 0.7; liver, 1.8 +/- 0.2; spleen, 1.4 +/- 0.4 (n = 5); muscle, 2.4 +/- 0.8 (n = 6); and meconium, 356 +/- 104. That the immunoreactive material measured behaved like hCG was determined by RIA of the supernatants. Parallelism was demonstrated between dilution curves for the tissue homogenates and the hCG standard for all tissues except meconium. A rat Leydig cell in vitro bioassay was used to demonstrate that there was hCG biological activity in the supernatants in ovarian, thymic, and renal tissues. The mean ratios of biological to immunological activities were 5.3 in kidney (n = 4), 1.6 in thymus (n = 3), and 1.3 in ovary (n = 2). Blood content of the tissues was determined from measurements of hemoglobin levels and it was found that for the ovary, testis, kidney, and thymus, hCG concentrations were higher than could be explained by the presence of circulating hCG in the tissues. These results, together with our previous results of the binding and effects of hCG in the human fetal testis, support the fact that the fetal testis is a target organ for hCG in the stimulation of steroidogenesis. The presence of high levels of hCG in the ovary, thymus, kidney, and meconium poses questions for further study of the possible physiological role of hCG.