The venous circulatory changes in 12 intubated premature infants with fluctuations or elevations in arterial BP were studied. The objectives of the study were to determine whether important alterations in venous pressure occur in infants with fluctuations in arterial BP or with elevations in arterial BP associated with suctioning. Venous and arterial catheters were present in all infants. Perfusion pressure was derived from the difference between mean arterial BP and venous pressure. At rest, minimal change in arterial BP, venous pressure, and perfusion pressure was observed. Elevations in arterial BP were accompanied by pronounced changes in venous pressure. However, because the magnitude and the direction of the changes in venous pressure were not consistent, pronounced changes in perfusion pressure resulted. Fluctuations in arterial BP were associated with fluctuations in venous pressure, which also resulted in pronounced and continuous alterations in perfusion pressure. Induced muscle paralysis in two infants obliterated the changes in arterial BP, venous pressure, and perfusion pressure associated with suctioning. These data demonstrate that marked venous circulatory changes accompany common arterial circulatory abnormalities; such alterations on the venous side of the circulation may be important in the pathogenesis of neonatal hemorrhagic and/or ischemic injury.