Current data suggest that the life spans of commonly held rodent species have decreased and the occurrence of tumors occurs earlier in the lifespan of ad libitum-fed animals. The most likely cause of this change in the life span of barrier-reared genetically defined animals is increased body weight. The effects of caloric restriction on a variety of functional measures and on age-dependent tumors and lesions are the focus of this presentation. Recommendations for caloric restriction, or at least "dietary control," will be discussed.