The contribution of the fetus to inhibin production in human pregnancy is unclear. Previous studies have reported inhibin concentrations in cord blood but on limited sample numbers. The present study is a more extensive examination of sex differences in immunoreactive inhibin and steroidal hormones in term umbilical cord blood. Venous serum concentrations of inhibin were found to be significantly higher in males (mean +/- SD 2,168 +/- 914 pg/ml, n = 62) than in females (1,761 +/- 951 pg/ml, n = 63) (p < 0.01). Male concentrations of total testosterone (p < 0.005), free testosterone (p < 0.005), and estradiol (p < 0.05) were also significantly higher. The concentration of dehydroepiandrosterone sulphate (DHEAS) and the sex hormone-binding globulin (SHBG) capacity were similar in males and females. Multivariate regression analysis showed that the inhibin concentration in males correlated with the testosterone concentration and in females it correlated not only with testosterone concentration but also with gestational age at delivery and SHBG capacity (all p < 0.001). Inhibin concentrations in cord arterial serum in a subgroup of 24 males and 31 females were higher than in the venous serum in both sexes but the differences were not statistically significant. Concentrations of DHEAS determined in 22 female cord arterial blood samples (6.0 +/- 2.5 mumol/l) were significantly higher than the venous concentrations (5.2 +/- 1.8 mumol/l) (p < 0.05). Inhibin did not correlate with DHEAS either in their arterial serum concentrations or in the arteriovenous concentration differences. The findings of this study are consistent with the placenta being the principal source of inhibin circulating in the human fetus in late pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)