We describe a method which allows longitudinal measurements of alveolar bone loss in the living rat. The anesthetized animal is kept in a fixed and reproducible position and radiographs of an upper molar arch are taken with a mammograph. In a preliminary study, 4 rats were repeatedly radiographed. In the main study, 10 rats were radiographed at day 0 and after 63 days of diet known to cause periodontitis. The radiographs were enlarged and the amount of bone lost calculated by comparing the weights of the interdental areas reproduced on standard paper. In the preliminary study, the coefficient of variation (C.V.) of the average weights of paper were fairly low (4 to 9%) in the first 3 rats, and amounted to 17% in the fourth. A statistically significant average loss of bone (16%) was found in the 10 rats in the main study when comparing the weights of papers at days 0 and 63. J Periodontol 1997;68:141–144.