We have shown that the major active agent of Cannabis sativa, Δ9-tetrahydrocannabinol, activates peroxisome proliferator-activated receptor gamma [PPARγ, O'Sullivan, S.E., Tarling, E.J., Bennett, A.J., Kendall, D.A., Randall, M.D., 2005c. Novel time-dependent vascular actions of delta9-tetrahydrocannabinol mediated by peroxisome proliferator-activated receptor gamma. Biochem. Biophys. Res. Commun. 337, 824–831]. The aim of the present study was to investigate whether another pharmacologically active phytocannabinoid, cannabidiol, similarly activates PPARγ. Functional vascular studies were carried out in rat aortae in vitro by myography. PPARγ activation was investigated using reporter gene assays, a PPARγ competition-binding assay and an adipogenesis assay. Cannabidiol caused time-dependent (over 2 h) vasorelaxation of pre-constricted aortae, sensitive to PPARγ antagonism (GW9662, 1 μM) and super oxide dismutase inhibition. The vascular effects of cannabidiol were not affected by endothelial denudation, nitric oxide synthase inhibition, pertussis toxin, cannabinoid CB1 or cannabinoid CB2 receptor antagonism, or capsaicin pre-treatment. When aortae were contracted with U46619 in a Ca2+-free buffer, vasorelaxation to cannabidiol was substantially reduced. Furthermore, cannabidiol (1–30 μM) inhibited the contractile response to the re-introduction of Ca2+. In a reporter gene assay, cannabidiol increased the transcriptional activity of PPARγ. Cannabidiol was also found to bind to PPARγ and stimulate the differentiation of 3T3-L1 fibroblasts into adipocytes, a PPARγ-mediated response. These results show that cannabidiol binds to and activates PPARγ, which partially underlies the time-dependent vascular effects of cannabidiol. However, cannabidiol-induced vasorelaxation in the rat isolated aorta appears to be largely due to calcium channel inhibition.