The contribution of carotid chemoreceptors to hypercapnia-induced mesenteric venoconstriction was examined in 12 -chloralose-anesthetized rabbits (1.0-1.6 kg). Surgical preparation consisted of a tracheotomy, femoral arterial and venous cannulation, and a midline laparotomy through which a 13-cm loop of ileum was exteriorized and superfused with physiological salt solution. Mesenteric vein diameter and intravenous pressure (using a servo-null measurement system) were measured in 500- to 1,000-µm mesenteric veins during 40-s periods of 15%, 20%, and 25% CO₂ inhalation. Measurements were then repeated following bilateral ablation of the carotid chemoreceptors. Before denervation, mesenteric vein diameter constricted 6.5 ± 1.1%, 11.9 ± 1.1%, and 17.9 ± 2.2% during the 15%, 20%, and 25% CO₂ inhalation, respectively. After denervation, these values were reduced to 5.0 ± 0.9%, 6.9 ± 1.2%, and 8.4 ± 1.3%, respectively. We conclude that activation of the carotid chemoreceptors by hypercapnia induces active mesenteric venoconstriction. After denervation of the carotid baroreceptors and chemoreceptors, there was also a small decrease in venule diameter proportional to the level of inspired CO₂. We further conclude that noncarotid body chemoreceptor activation contributes to mesenteric venular constriction.