Cortisol and corticosterone were determined in mixed umbillical cord plasma of 43 healthy full-term newborns. The method consisted of a combined thin-layer chromatographic-fluorimetric procedure which proved to be specific and reliable. The mean concentration in cord plasma of cortisol was 10.6 plus or minus 4.9 mug/100 ml, of corticosterone 1.8 plus or minus 0.8 mug/100 ml. The mean ratio cortisol/corticosterone F/B was 6.3 plus or minus 2.5. Neither the duration nor the time of day of delivery appeared to influence the concentration of cortisol or corticosterone in umbilical cord plasma. Also, there was no significant difference between male and female infants. In 18 instances of a pathological course of gestation and/or delivery the mean cortisol level was 9.1 plus or minus /.7 mug/ml, the mean corticosterone level 2.2 plus or minus 9 mug/100ml. The mean F/B ratio was slightly but not significantly decreased (4.2 plus or minus 1.4 mug/100 mo; p larger than 0.05). It is speculated that the high corticosterone concentration in umbilical cord plasma reflects a defect in cortisol biosynthesis (17 chi-hydroxylase deficiency) in the newborn, compared with later life.