New apparatus was made whereby indices of cerebral oxygenation and haemodynamics in sick newborn infants could be quantified by near infrared (NIR) spectrophotometry and displayed instantaneously at the cotside. The indices included oxygenated haemoglobin, reduced haemoglobin, oxidised cytochrome aa3, and total haemoglobin concentration: cerebral blood volume, mixed cerebral venous saturation, and changes in cerebral blood flow were then derived. Striking changes were observed in response to alterations in arterial oxygen saturation and carbon dioxide tension and to tilting of the infant. Abnormal responses were detected in cerebral oedema following birth asphyxia, patent ductus arteriosus, and cystic encephalomalacia. NIR spectrophotometry provides valuable quantitative data at the cotside for the management of sick infants and for exploring the pathophysiology of damage to the brain.