Summary

Spontaneous age-associated patterns of neoplastic and nonneoplastic lesions were studied in 670 rats, and survival data were compiled on 791 rats that completed their normal life spans. Rats were of the Brown Norway (BN)/Bi and Wistar Strains (WAG)/Rij and their (WAG × BN)F₁ hybrid and included both males and females. The life span study showed that male and female BN/Bi, male and female (WAG × BN)F₁, and female WAG/Rij rats had 50% survival ages of about 30 months. Major differences were found when the percentage or incidence of lesions in rats that died spontaneously was compared to rats that were killed at selected ages. The percentage of some lesions in killed rats was closest to that found in the population at risk as calculated by life table techniques, whereas the incidence of other lesions in the killed rats was closest to that found in animals that had died spontaneously.