The transcription factor PPARgamma is expressed in endothelium and vascular muscle where it may exert antiinflammatory and antioxidant effects. We tested the hypothesis that PPARgamma plays a protective role in the vasculature by examining vascular structure and function in heterozygous knockin mice expressing the P465L dominant negative mutation in PPARgamma (L/+). In L/+ aorta, responses to the endothelium-dependent agonist acetylcholine (ACh) were not affected, but there was an increase in contraction to serotonin, PGF(2alpha), and endothelin-1. In cerebral blood vessels both in vitro and in vivo, ACh produced dilation that was markedly impaired in L/+ mice. Superoxide levels were elevated in cerebral arterioles from L/+ mice and responses to ACh were restored to normal with a scavenger of superoxide. Diameter of maximally dilated cerebral arterioles was less, whereas wall thickness and cross-sectional area was greater in L/+ mice, indicating cerebral arterioles underwent hypertrophy and remodeling. Thus, interference with PPARgamma signaling produces endothelial dysfunction via a mechanism involving oxidative stress and causes vascular hypertrophy and inward remodeling. These findings indicate that PPARgamma has vascular effects which are particularly profound in the cerebral circulation and provide genetic evidence that PPARgamma plays a critical role in protecting blood vessels.