Scrum prolactin values in normal pregnant women showed a progressive increase from a mean value of 50 ng/ml in the 12th week to 270 ng/ml at term, with the range at term being 100-600 ng/ml. There was a fairly good correlation (r=0.7) between the values of 24-hour urine estriol in 138 determinations and in the serum prolactin in 133 pregnant women. The regression lines of serum hPRL values with time of gestation in cases of intrauterine growth retardation (IUGR) and diabetes mellitus were less steep than those seen in normal pregnancy. The serum hPRL value of patients with preeclamptic toxemia, latent diabetes, premature rupture of membranes, or multiple pregnancies were found not to differ significantly from the values observed in normal pregnancy. The results indicate that prolactin determinations in pathologic pregnancies are not useful as an aid in their evaluation.