Individuals with epilepsy also often exhibit symptoms of attention deficit hyperactivity disorder (ADHD). The ketogenic diet, which is a high fat, low protein, and low carbohydrate diet used in the treatment of intractable epilepsy, also appears to improve symptoms of ADHD in individuals with both disorders. Previous research suggests that the diet decreases the activity level of rats. The purpose of the present research was to further investigate the effects of the ketogenic diet on activity level, using an animal model. Two experiments were conducted. The first experiment examined the time frame and reversibility of the effect of the diet on activity level. The second experiment examined the relationship between activity level and anxiety level. In both experiments, adult male Long-Evans rats were placed on either a ketogenic diet or a control diet. The results of the first experiment show that the ketogenic diet can cause a decrease in activity level within 24 h and that the results are reversible. The results of Experiment 2 show that the decrease in activity level is not linked to a change in anxiety level. The ketogenic diet may be of use in the treatment of ADHD.