High-fat diet has been shown to increase expression of uncoupling protein 3 (UCP3) in genetic and non-genetic obesity and has been implicated in the decreased cardiac efficiency (1). Apolipoprotein E knockout (apoE-/-) mice fed high-fat for 6 months, are not obese and develop atherosclerotic lesions (2). In contrast to their male counterparts, female apoE-/- mice on high-fat diet do not show evidence of occlusive coronary lesions or myocardial infarction (3). The aim of this work was to investigate whether high-fat diet alters cardiac UCP3 expression in this model and whether there is a difference between male and female mice. Female and male apoE-/- mice (mixed background C57Bl/6 and 129) at 8 weeks old were either switched onto high fat, Western-type diet (21% fat; 0.15% cholesterol) or were maintained on normal rodent diet for approximately 6 months. Myocardial tissue was collected from male (n=6) and female (n=6) mice and proteins were extracted as described previously (3). Western Blotting was used to estimate UCP3 expression in cardiac tissue using a polyclonal rabbit anti-UCP3 antibody (Abcam plc) and data expressed as a ratio of UCP3 band intensity to GAPDH using Image J. Data are presented as mean±SE and differences determined using unpaired t-test (n=3 for each group). High-fat diet was found to significantly increase UCP3 expression in male heart (1.11±0.19 vs. 2.04±0.15, p<0.05) but not in female heart (0.66±0.15 to 0.81±0.16). There was no significant difference in UCP3 expression between male and female heart on normal diet (1.11±0.19 vs. 0.66±0.15). This work demonstrates that high-fat feeding increases UCP3 expression independent of obesity. Furthermore, the increase of UCP3 expression in heart appears to be associated with coronary disease as female hearts on high-fat diet without evidence of coronary disease do not show significant increase in UCP3 expression.