In this study, in order to clarify the kinetics of leptin, we focused on the ratio of leptin concentrations in cerebrospinal fluid and serum in aged male rats, and examined the weight of epididymal fat, and the passage rate of leptin through the blood-brain barrier. In the lighter animals, the epididymal fat weight was low, while leptin concentrations in the serum and cerebrospinal fluid were also low. Conversely, in the heavier animals, the weight of epididymal fat and leptin concentrations in the serum and cerebrospinal fluid were higher. With regard to the ratio of leptin in the cerebrospinal fluid and serum, the passage rate of leptin through the blood-brain barrier was lower in the heavier animals than in the lighter animals.