The 17 alpha-hydroxyprogesterone (17-HP) levels of cord and peripheral blood plasma were determined in fullterm and in preterm neonates, in order to investigate the normal concentration range during the first week of extrauterine life. In both fullterm and preterm neonates, the 17-HP level of the peripheral blood is substantially lower than that of the cord blood. In fullterm neonates (aged 1-7 days) the mean 17-HP concentration of the peripheral blood is higher in boys than in girls; the difference is significant (p less than 0.05). In preterm neonates, however, the 17-HP level of the cord blood is higher for girls than for boys, the difference again being significant (p less than 0.05).