High dietary fat intake is considered to be a factor in the development of obesity. However, evidence suggests not all dietary fats are equally obesegenic. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have been suggested to stimulate processes that reduce body weight and specifically prevent the deposition of visceral adipose tissue in rodent models. However, these findings are inconsistent and the comparator fats used have not been representative of typical Western dietary fats. This study was designed to determine whether rats fed a high fat diet based on fish oils, high in EPA and DHA, were less likely to develop obesity than those fed on a diet based on anhydrous milk fat (rich in the saturated and monounsaturated fatty acids common in the Western diet). Male Wistar rats were ad libitum fed high fat diets with 40% of energy from fish oil (HF-FO) or anhydrous milk fat (HF-AMF) (n=9-10 per group) for 12 weeks. Diets were matched for energy density, protein, carbohydrate and fat content. Food intake and body weights were measured daily. At the end of the study, trunk blood was taken by cardiac puncture for the determination of plasma thyroid hormone concentrations. Uncoupling protein-1 (UCP-1) mRNA expression was measured in brown adipose tissue by Northern blot. Magnetic resonance imaging (MRI) was used to assess adipose tissue volumes in subcutaneous and visceral depots (n=5 per group). Generalised estimating equations were used to compare body weight and energy intake changes between groups and unpaired t-tests used for all other data. The HF-AMF group had a significantly increased energy intake compared to rats fed the HF-FO diet (p<0.001). Despite this difference in energy intake, there was no significant difference in body weight between the groups over the course of the study (p=0.80). Similarly, there was no significant difference in total, visceral or subcutaneous adipose tissue between the two groups (p=0.50, p=0.15 and p=0.92 respectively). UCP-1 mRNA expression was not significantly different between the two groups (p=0.99). Circulating thyroxine (T4) was found to be significantly lower in the HF-FO diet group (p=0.04) although no difference was observed in tri-iodothyronine (T3) concentrations (p=0.39). These data suggest that a HF-AMF diet may promote energy overconsumption compared to an isocaloric HF-FO diet, but this does not appear to be associated with an increase in body weight or adiposity. This may suggest a greater metabolic efficiency in the HF-FO group, or a decreased energy expenditure (perhaps indicated by the reduced T4). It is necessary to measure energy expenditure to confirm this. Although Western diets rich in fats such as AMF have been hypothesised to favour the development of obesity compared to diets rich in EPA and DHA, this data suggests that this was not the case in this model.