A study was conducted to determine the correct interval for rabies vaccination. Serum antibody titer was used to assessed for prevalent vaccination programs. 155 apparently healthy dogs were selected from a group of animals presented for initial vaccination to the veterinary clinic at the University of Tehran. These dogs were divided into three age groups; under four months, four months to five years, and above five years of age. Serum was taken on the day of referral, and at 1, 3, 6, 9, and 12 months after vaccination with cerebral vaccine from the Pasteur Institute. The level of antibody in the sera was measured with the RFFIT method. The results showed that vaccine prepared on mouse brain cell lines causes weak immunity titer for a maximum of six months. The best immune response was in animals three months of age, and the least response in the animals above five years old. We conclude that a second vaccine at 6 months of age is needed to preserve the immunity at an optimal level. The other notable point is that in countries such as Iran where rabies is endemic and there is zoonotic danger for humans, it is better to induce initial immunization with a multi-dose protocol followed by annual boosters. Finally, it should be mentioned that due to the importance of disease in pups, it is better to vaccinate in the 2nd month of age, as with the other viral vaccines.