Levels of cortisol were determined in umbilical cord serum in the presence and absence of labor at various gestational ages. In the absence of labor, serum cortisol fell (P less than 0.025) from 8.4 ng/ml at 15 to 17 weeks to 4 ng/ml at 17 1/2 to 20 weeks, a change which coincides with a rapid drop in relative adrenal weight. By 35 to 36 weeks, the mean levels had risen to 20 ng/ml and there was a further increase between 37 1/2 and 40 weeks (P less than 0.01) to a mean level of 45.1 ng/ml. Data obtained at vaginal delivery after the spontaneous onset of labor over the period of 26 to 40 weeks were more variable but showed a similar pattern, with levels about twice those in the absence of labor. No differences could be attributed to sex or to the anesthetic used. Thus, there appears to be a rise in the fetal level of cortisol with gestational age in late pregnancy, whether or not labor takes place. This rise is steepest immediately before the normal time of onset of labor and cannot be attributed to the stress associated with labor.