Concentrations of prolactin (PRL), estradiol (E2), thyroxine (T4), triiodothyronine (T3), and reverse T3 in umbilical cord blood were measured by radioimmunoassay in neonates with (n = 60) and in a control group without (n = 345) hyaline membrane disease. Mean levels of all hormones assayed were not significantly different between the two groups at various stages of gestation. In the control group, gestational age correlated positively with PRL and inversely with reverse T3, whereas birth weight correlated positively with PRL, T4, and T3, but inversely with reverse T3 levels. Thus, larger, more mature neonates tended to have higher cord levels of PRL, T4, T3 and lower concentrations of reverse T3. The data also suggest that, in the premature neonate, various obstetric complications and exposure in utero to beta-mimetic drugs and glucocorticoids may be important determinants of the concentrations of E2 and thyroid hormone in cord blood.