Blood samples were obtained from the maternal peripheral vein (n = 17) during the second stage of spontaneous labor and from the umbilical artery (UA) and vein (UVn) immediately after delivery. Four delta 4-and five delta 5-steroids as well as estradiol were measured by RIA. Additionally, cortisol was determined in maternal plasma. The maternal concentrations were compared with our control values obtained from samples (n = 30) collected at term but not in labor. All delta 5-steroids and F were significantly higher during labor than before labor. In the umbilical circulation, progesterone and 17-hydroxyprogesterone were significantly higher in the UVn than in UA (P < 0.05). Whereas androstenedione was significantly higher in the UA [1.01 +/- 0.12 ng/ml (mean +/- SEM)] than in the UVn (0.72 +/- 0.07 ng/ml), testosterone was almost identical in the two (UA, 0.33 +/- 0.03 ng/ml; UVn, 0.35 +/- 0.08 ng/ml). Four of the five delta 5-steroids (dehydroepiandrosterone, dehydroepiandrosterone sulfate, pregnenolone sulfate, and 17-hydroxypregnenolone) had significantly higher concentrations in the UA than in the UVn (P < 0.05). However, pregnenolone levels were not significantly different (UVn, 21.8 +/- 2.0 ng/ml; UA, 18.0 +/- 2.0 ng/ml). No significant correlations were observed between maternal and umbilical steroid concentrations.