Mean beta-endorphin-like immunoreactivity in the plasma of 10 normal women in the 10th month of pregnancy was 144.3 +/- 7.5 ng/l. During labor in 7 women its immunoreactivity was increased and peaked at the time of vaginal delivery (1 162 +/- 69 ng/l). Two hours after delivery, beta-endorphin-like immunoreactivity was significantly decreased (297 +/- 39 ng/l) and after 4 to 5 days was 155 +/- 33 ng/l. Beta-endorphin-like immunoreactivity in the cord plasma (523 +/- 30 ng/l) was significantly lower than in the mother at the time of vaginal delivery and in venous blood of newborns 24 hours post partum was 156 +/- 11 ng/l. The correlation between beta-endorphin-like immunoreactivity in the mother and the cord blood plasma was not determined. At the time of the fetal hypoxia, beta-endorphin-like immunoreactivity in the cord blood plasma was increased (2 741 ng/l). We conclude that immunoreactive beta-endorphin influences a stress reaction in the mother and fetus at the time of labor. During intra-uterine life the fetus probably produces its own immunoreactive-like beta-endorphin independently of the maternal production of this peptide.