To determine if crying alters cerebral hemodynamics and oxidative metabolism in the brain, near infrared spectrophotometry was used to assess relative changes in cerebral blood volume and the oxidation-reduction state of cytochrome aa3. Thirty-six crying episodes were observed, 20 in healthy infants and 16 in infants with respiratory problems. Throughout all crying episodes cerebral blood volume and oxidized cytochrome aa3 demonstrated oscillatory fluctuations every 10 to 20 seconds, with maximum changes during prolonged exhalations. In 86% of episodes baseline blood volume rose and remained elevated during the cry. The relative content of deoxyhemoglobin in cerebral blood also rose, indicating that venous blood is the major contributor to the increase in blood volume. Changes in baseline cytochrome aa3 oxidation varied with the presence of lung disease and with the chronologic age of the infant. Cytochrome reduction with crying occurred significantly more often in infants with respiratory problems than in healthy infants. Cytochrome aa3 became more oxidized in 82% of crying episodes in healthy infants older than 3 days of age, but no change in cytochrome oxidation was usually noted in those younger than 3 days. Thus crying alters cerebral blood volume in all neonates in a pattern consistent with cyclic obstruction to cerebral venous return; it decreases cerebral oxygenation in infants with respiratory problems.