Previous studies in adults have shown that chronic pulmonary hypertension is associated with decreased endothelial nitric oxide synthase (eNOS) expression in pulmonary arteries. However, the role of decreased eNOS expression in persistent pulmonary hypertension of the newborn (PPHN) is unknown. We investigated the hypothesis that umbilical vein endothelial cells cultured from infants with PPHN will have decreased eNOS expression. Umbilical cords were collected from meconium-stained infants at birth, and endothelial cells were isolated if the infants developed PPHN. Endothelial cells were grown in primary culture, and total RNA was isolated. cDNA was reverse transcribed from mRNA and amplified by PCR. An expected product of approximately 550 bp was found in all control infants but only in two of the six infants with PPHN. Identity of the PCR product was confirmed by Southern hybridization to a separate internal eNOS-specific probe. Amplification of beta-actin cDNA, an internal control, was detected in all controls and in all infants with PPHN, including the four infants without the eNOS band. There was no difference in the course and outcome of patients with presence or absence of the eNOS band. However, there was an acidotic arterial blood pH (7.19-7.29) and intrapartum fetal heart rate decelerations in all four infants without eNOS expression. In conclusion, eNOS mRNA was detected in all normal term infants but was notably absent in the majority of infants with PPHN in this pilot study. The development of PPHN is multifactorial, and a decrease in eNOS gene expression may occur in some infants. Whether the decreased eNOS transcript is a cause of PPHN or a result of intrapartum stress remains to be determined.