Maternal plasma prolactin, estradiol-17 beta, and free estriol levels were estimated in 118 normal pregnancies and in 90 patients with preeclampsia at 35 to 40 weeks' gestation. In the preeclamptic patients, the prolactin values were similar to those of normal pregnancy; the levels were not affected by the severity of disease. At 37 to 38 weeks' gestation the maternal plasma free estriol levels correlated positively with the prolactin levels in preeclampsia (r = .313; P less than .05; N = 64). No correlation was found between the levels of prolactin and estradiol-17 beta or between prolactin and urinary estrogen. Fetal distress was related to decreased plasma estradiol levels, but this was not reflected in the prolactin concentrations. The results of the present study show that the estimation of plasma prolactin level is of no clinical significance in patients with preeclampsia.