The oral toxicity of gamma-cyclodextrin (gamma-CD) was examined in a 13-week feeding study in which four groups of four male and four female Beagle dogs received gamma-CD in the diet at concentrations of 0 (control), 5, 10, or 20%. No treatment-related changes were noted in behavior or appearance of the dogs and no mortalities occurred. Transient diarrhea occurred in some dogs of the 5 and 10% dose groups and in all dogs of the 20% dose group. However, all dogs remained in good health and gained weight. During the last 6 weeks of the study, the males of the 20% dose group gained less weight, but body weights were not significantly reduced in comparison to controls. Food intakes and food efficiencies were comparable among all groups. No treatment-related differences were observed with respect to ophthalmoscopic examinations, hematological parameters, clinicochemical analyses of the plasma, and semiquantitative urine analyses. Only the urinary pH was slightly below control levels in males of the 20% dose group. No abnormalities were seen at necropsy that could be attributed to treatment. The organ weight data revealed some cecal enlargement in the 10 and 20% dose groups. Relative ovary weights were significantly increased in the 10 and 20% groups but this was probably a result of an unusually low ovary weight in the controls. An increase of relative liver weights in males of the 10 and 20% dose groups also was considered to lack toxicological relevance because it was not associated with changes in plasma liver enzyme levels or histopathological changes. On microscopic examination, no treatment-related effects were observed in any of the various organs and tissues. In conclusion, transient diarrhea, cecal enlargement, and a slightly increased acidity of the urine were the only treatment-related effects reported. These changes are well-known physiological responses to the presence of increased amounts of undigested, fermentable carbohydrates in the lower gut. At the high applied intakes an incomplete digestion of gamma-CD and/or a partial inhibition of pancreatic amylase by gamma-CD could account for these effects. It is concluded that daily gamma-CD consumption of up to 20% in the diet (approximately 7.7 g/kg body wt in male and 8.3 g/kg body wt in female dogs) was tolerated without any toxic effects.