In the discussion about policies and strategies for rabies prevention in developing countries, intervention costs arise as a major issue. In a pilot mass vaccination campaign against rabies in N'Djaména, Chad, 3000 dogs were vaccinated. We assessed vaccination coverage and cost, showing the cost per dog vaccinated for the public sector and for society. An extrapolation to city level calculated the approximate cost of vaccinating all 23 600 dogs in N'Djaména. In the pilot mass campaign with 3000 dogs the average cost per dog was 1.69 euro. to the public and the full societal cost was 2.45 euro. If all 23 600 dogs in N'Djaména were vaccinated, the average cost would fall to 1.16 euro to the public and 1.93 euro to society. Private sector costs account for 31% of the cost to vaccinate 3000 dogs, and 40% of the cost to vaccinate 23 600 dogs. Mass dog vaccination could be a comparatively cheap and ethical way to both control the disease in animals and prevent human cases and exposure, especially in developing countries. The cost-effectiveness of dog vaccination compared with treating victims of dog bites for prevention of human rabies should be further assessed and documented.