The locations of heme oxygenase-2 (HO-2) and heme oxygenase-1 (HO-1) in the walls of 1st- to 4th-order arterial branches in the pia mater were studied in normotensive rats and in animals with induced renovascular hypertension (RVH). Immunohistochemical detection of heme oxygenase was performed on 40 Wistar rats. The vasomotor function of carbon monoxide (CO) in 3rd- to 5th-order arteries was found to be associated mainly with the endothelium, while in 1st- and 2nd-order arteries it was associated with myocytes. In normotensive animals, HO-2 was involved in forming CO, while both HO-2 and HO-1 had roles in hypertensive animals. The largest number of vessels expressing HO-1 was seen in week 16 of RVH, when the proportion of vessels positive for HO-2 was minimal, which is consistent with the time at which arterial pressure stabilizes at the new, higher, level.