Anaesthetized mechanically ventilated newborn rabbits were exposed to different degrees of hypercapnia. One hour of normocapnia was used as a control period. Renal function studies demonstrated an increase in renal vascular resistance with a concomitant decrease in effective renal plasma flow in all hypercapnic animals, combined with a less pronounced decrease in glomerular filtration rate. Filtration fraction rose significantly. A decrease in systemic blood pressure was only observed when the PaCO2exceeded 100 mm Hg combined with an arterial pH below or equal to 7.10. We conclude that normoxemic hypercapnia in the newborn rabbit leads to an increase in renal vascular resistance and suggest that the renal vasoconstriction predominates at the level of the efferent arteriole.