Volume expansion is one of the most frequent used interventions in critically ill newborns, despite lack of hard evidence. In a case of a truly hypovolaemic patient, for example after massive hemorrhage secondary to an abruptio placentae, volume expansion is life-saving. However, volume expansion in a normo- and/or hypervolemic newborn infant is not without risk. Excessive fluid intake is associated with a disturbed neurologic outcome, an increased prevalence of chronic lung disease and an increased mortality. It would be profitable when the volume status of a critically ill newborn infant could objectively be assessed so the response to volume expansion could be predicted (‘fluid responsiveness’ - FR). The clinical assessment of the volume status is rather unreliable. Recently new dynamic parameters are introduced that reliably predict fluid responsiveness in ventilated adult patients. These variables, like ‘pulse pressure variation’ (PPV) and ‘systolic pressure variation’ (SPV), are based on arterial blood pressure variations secondary to mechanical ventilation (heart-lung interaction).This inspired us to study the phenomenon of arterial blood pressure variation in critically ill newborn infants under several clinical conditions. The (preliminary) data will be presented.