Caries, periodontal disease, and occlusal wear were studied in rats with short-term (one month) and long-term (12 months) untreated alloxan diabetes and in age-matched control rats. Standard laboratory diet and water were given ad libitum. Streptococcus mutans and lactobacilli were naturally occurring inhabitants of the oral bacterial flora. In short-term diabetic rats, there was less dental plaque in the gingival region of the first mandibular molar than in control rats, but the density of leukocytes in the junctional epithelium was increased. Interdentally, the gingival papillae were intact, and the prevalence of foreign material was as low as for the controls. No caries lesions were seen. The proportion of the oral flora which was lactobacilli was positively correlated with the blood glucose level. In long-term diabetic rats, there was more periodontal breakdown than in the age-matched control rats. The periodontal breakdown was always associated with large interdental impactions of foreign material. Caries was seen only in the diabetic rats and in sites with interdental impactions. The decay was shown to start in the cementum at the cemento-enamel junction. The amount of alveolar bone loss, but not the amount of dental caries, was related to the degree of hyperglycemia. Occlusal wear of the molars in long-term diabetic rats was significantly increased as compared with that in the controls.