It is sometimes difficult to assess the relevance of tumors that occur in treated animals in short-term studies. This report is intended to establish a general profile of tumor occurrence in young control CD-1 mice and Sprague-Dawley rats. Data from 20 rat and 20 mouse carcinogenicity studies conducted between 1990 and 2002 at Huntingdon Life Sciences, UK. were collected and evaluated. The route of administration was either dietary oral gavage, and the analysis was confined to sporadic deaths (decedents) in control groups occurring during the first 50 weeks of study. In addition, tumor occurrence between 50-80 weeks were compared. In mice, the most common tumor was lymphoma, followed by bronchiolo-alveolar adenoma. In rats, the most common tumor was adenoma of the pituitary gland, followed by mammary fibroadenoma, and adenocarcinoma. When studies of up to 50 weeks, between 50 and 80 weeks, and at 2-year termination were compared, there was no great difference in tumor occurrence except in male rats, in which the most common tumor up to 50 weeks on study was lymphoma, whereas the most common tumor between 50-80 weeks and at 2 years was pituitary adenoma.