Levels of testosterone (T) (total and free), androstenedione (A4), dehydroepiandrosterone sulphate (DHEAS), sex hormone-binding globulin (SHBG), and estradiol (E2) were measured by radioimmunoassay (RIA) in 156 normal pregnancies (77 male and 79 female fetuses). Samples were obtained from amniotic fluid, 2nd and 3rd trimester maternal serum, and umbilical cord serum at birth. During the critical period of brain differentiation, at the beginning of the second trimester of pregnancy, sex differences in T and A4 were found in amniotic fluid and not in maternal serum. This finding adds to the fact that mostly low and nonsignificant correlations were found for the different androgenic hormones between levels assessed in amniotic fluid and maternal plasma at this particular and very sensitive period of fetal brain development. On the other hand, high correlations were found for the same hormones between the samples of maternal serum in the 2nd and the 3rd trimester. Our data show that, of all available sources, amniotic fluid seems to be the best candidate to investigate the effects of early fetal androgen exposure. © 2004 Elsevier Inc. All rights reserved.