Corn oil is well tolerated by mice but tolerance may decrease with excessive ingestion. In the present study, we compared the effects of optional ingestion of excessive corn oil with ingestion of water (control) or a 20% sucrose solution in mice. During the entire study, mice consistently ingested 100% corn oil and incrementally ingested 20% sucrose. Food intake in the corn-oil group was approximately constant but that in the sucrose group was slightly decreased. Body-weight gains in the corn-oil group were higher than those in the control and sucrose groups. At the end of the study, hepatic hypertrophy and fatty liver were present, especially in the corn-oil group, and the visceral fat of mice fed corn oil increased significantly compared with the other two groups. These results suggest that mice, when given a choice, will continue to overeat corn oil over the long term, inducing excessive caloric intake and obesity.