Using mice transgenic for the growth hormone gene (TGHM), we have studied the effects of a systemic elevation of growth hormone on vascular growth with the aim of investigating the role of vascular mass changes in producing hypertension. In contrast to human acromegaly or gigantism, there was no elevation of blood pressure in TGHM, but there were significant increases in vascular wall mass. In accordance with a presumably Increased perfusion of larger organs, the medial cross-sectional areas of thoracic aorta and mesenteric resistance vessels were greater in the TGHM. These differences could be normalized in the aorta by body weight and in the mesenteric vessel by small intestine weight. Furthermore, the brain was not significantly heavier in the TGHM, and their carotid and cerebral vessels also were not larger. Wallto- lumen ratios were similar in the aorta, carotid, and middle cerebral arteries suggesting that wall stress was the controlling factor in wall thickness. Surprisingly, the mesenteric vessels had increased wall-to-lumen ratio, which was similar to that seen in hypertensive vascular remodeling but in a normotensive animal. In an attempt to explain this finding it was noted that the pattern of mesenteric vascular networks and even organized structure within the vessel wall itself appeared to be fixed, perhaps by genetic mechanisms. Thus, vascular network structure may be a potentially limiting factor in the ability of the vessel wall to remodel and may have been responsible for the greater wall-to-lumen ratio in TGHM mesenteric vessels. A similar situation in human acromegaly or gigantism could result in a circulation marginally able to correct for other demands on blood flow resulting in about one third of cases being hypertensive.