Marked fluctuations of arterial blood pressure (ABP) are associated with an increased risk of intraventricular hemorrhage. The pathophysiology is linked to spontaneous breathing. We hypothesized that these fluctuations would decrease after initiation of effective mechanical ventilation. We studied 20 infants treated with nasal continuous positive airway pressure (CPAP) and oxygen for early respiratory distress. Eleven required intubation for clinical indications. Simultaneous ABP and respiratory tracing obtained before and following intubation were analyzed for average peak systolic blood pressure (SBP) and the percentage of spontaneous respirations (SResp). Fluctuations of SBP were quantitated using the coefficient of variation (CV). The remaining 9 infants with minimal lung disease served as a control group. There was a small increase in SBP, and a marked decrease in both CV and SResp following intubation. A significant correlation was found between the percent decreases in CV and SResp. We conclude that beat-to-beat fluctuations of ABP decrease after the initiation of effective mechanical ventilation. This effect is primarily due to a decrease in spontaneous breathing.