A 90-day ad libitum administration toxicity study of oligoglucosamine (OG) was carried out using F344 rats of both sexes. The animals were divided into four groups of 20 animals each, 10 of each sex, and fed a diet containing 0, 0.04, 0.2 or 1.0 (w/w)% OG. During the administration period, no animals of either sex died or exhibited abnormal signs in the 0.04% OG and 0.2% OG groups. In the 1% OG group, in both sexes, erythema and swelling of the snout and forelimbs and loss of fur in the forelimbs were observed. On macroscopic observation, emaciation, swelling of the snout, auricles and forelimbs and alopecia of the forelimbs were also observed in 2-3 males of the 1% OG group. It was suggested that these topical abnormalities might be due to dermal responses to OG adhering to the skin and fur, which are easily soiled with saliva during grooming. In the animals of the 1% OG group, food consumption decreased, resulting in body weight gain being suppressed. This was found concomitantly with the abnormal findings mentioned above. Thus, feeding difficulties due to the topical lesions on the snout and forelimbs were thought to affect body weight. In hematology, platelet count, lymphocyte count and differential neutrophil count increased in males of the 1% OG group. These changes might be related to the dermal inflammation. Abnormalities in urinalysis and blood chemistry, as well as a small thymus, small spleen, dark spots or areas on the glandular stomach mucosa, pale Harderian glands and small testes in histopathology, were also observed in males in the 1% OG group. Whether or not all these changes were related only to the malnutrition remains to be elucidated. From these results, OG gave rise to no adverse effects in rats up to the dose level of 0.2 (w/w)%. Thus, the no observed adverse effect level was determined to be 0.2 (w/w)% for rats of either sex (124.0mg/kg/day in males, 142.0mg/kg/day in females).