Obesity and associated excess adipose tissue around the heart and vasculature are major risk factors for atherosclerotic cardiovascular disease (CVD)1. Perivascular adipose tissue (PVAT) surrounding the aorta is phenotypically brown and thermogenic in nature as demonstrated by the presence of numerous multilocular adipocytes, dense mitochondria and expression of uncoupling protein (UCP)1. Most rodent studies on adipose tissue function are typically carried out at 20-22°C which is well below their thermoneutral zone of 27-30°C2,3, making metabolic and cardiovascular studies difficult to interpret as UCP1 will be activated. This could be especially important as murine PVAT has previously been described as resistant to HFD4 under these conditions, as following 13 weeks HFD there was no evidence of lipid accumulation or inflammation in this depot. The present study was designed to examine the hypothesis that the impact of dietary induced obesity on PVAT would be dependent on housing temperature. Twenty four 3 week old female Sprague-Dawley rats were randomised (n=6 per group) to one of four groups: maintained at a standard housing temperature (~20°C) and fed a chow or obesogenic diet or maintained at thermoneutrality (~27°C) and fed a chow or obesogenic diet. At 10 weeks of age animals were euthanised by a Schedule 1 procedure in line with local and national procedures. Brown, visceral (peri-renal, gonadal and mesenteric) and subcutaneous (inguinal) adipose tissues were dissected and weighed whilst the aorta with PVAT attached was removed, fixed and processed prior to staining with H&E for the distribution of white and brown adipocytes. Values are means ± S.E.M. with significance determined by one-way ANOVA. Despite no difference in final body weight between groups HFD increased weight gain at thermoneutrality (147.6±14.61 vs. 177.4±7.25g; p=0.037) whilst an increased ambient temperature reduced weight gain in chow fed animals (175.9±3.9 vs. 147.6±14.6g; p=0.046). Subcutaneous fat mass increased following HFD at 20°C (0.31±0.06 vs 0.51±0.05%; p=0.003) and at 27°C (0.38±0.02 vs 0.6±0.04%; p=0.002) yet visceral mass increased only following HFD at 20°C (1.55±0.15 vs. 2.12±0.13%; p=0.037) with no change in BAT mass between groups. Histological analyses of PVAT demonstrated that a brown phenotype was maintained at 20°C even following exposure to an obesogenic diet (Fig. 1A and B). At thermoneutrality, however, PVAT exhibited a ‘whiter’ phenotype which was exacerbated by obesity as evidenced by larger unilocular white adipocytes (Fig. 1C and D). At thermoneutrality PVAT is ‘whiter’ and accumulates lipid during obesity, an effect not seen at sub-thermoneutrality. These temperature dependent differences in phenotype highlight the importance of housing at thermoneutrality for pre-clinical research.