Recent evidence shows an association between obesity and cognitive decline. The present study aimed to determine whether a very high fat (60%) or western diet can affect working or spatial memory in rats and whether the diet-induced cognitive impairment is linked to the level of acetylcholine in the brain. Three groups of male Long Evans rats were fed either chow, western diet (21% fat, 0.15% cholesterol) or a high fat diet (60% fat) for 12 weeks (n=12 per group). Body weight, food intake and blood pressure were measured weekly. Behavioural testing, novel object recognition and Y-maze were carried out at 12 weeks. At the end of the study brain choline acetyltransferase and acetylcholinesterase levels were estimated. Results showed that consumption of a western diet for twelve weeks impaired a rat's spatial memory (p<0.05), and increased body weight, calorie intake, blood pressure and triglyceride levels. Conversely our high fat diet also impaired spatial memory (p<0.05) but this effect was independent of the rat's body weight or blood pressure. No significant changes in brain acetylcholine markers were observed. In conclusion, diets with higher fat content impaired hippocampal-dependant memory, even when hypertension and obesity are absent; however the mechanism is still unclear.