1. The goal of this study was to examine the regulation of resistance in the large arteries and small vessels that supply the brain stem. 2. We used a new method in anaesthetized cats to measure blood flow to the medulla (microspheres) and pressure (servo‐null) in branches of the basilar artery that supply the medulla. Resistance was determined during normocapnia, hypercapnia, hypocapnia and seizures (produced with intravenous bicuculline). 3. Pressure in arteries that supply the medulla (150 microns internal diameter) was 71 +/‐ 4% (mean +/‐ S.E. of mean) of aortic pressure and large artery resistance was 31 +/‐ 4% of the total resistance in the medulla. Hypercapnia and seizures decreased large artery resistance by 67 and 50%, respectively, and hypocapnia increased large artery resistance by 58%. Small vessel resistance decreased by 82% during hypercapnia and by 43% during seizures, and increased by 96% during hypocapnia. 4. Thus, resistance of the large arteries (greater than 150 microns diameter) accounts for about one‐third of the total vascular resistance in the brain stem. Both large arteries and small vessels respond to alterations in arterial carbon dioxide tension and seizures, and contribute to the regulation of blood flow to the brain stem.