In a longevity study with SPF rats of the Han:Wistar outbred stock, 320 virgin males and 320 virgin females of marked littermates were maintained in a barrier-type animal house under highly standardized conditions from weaning until their natural end of life. Diseased and dead animals were sectioned and examined using histological, bacteriological and virological methods. The mortality of the rats is low up to the 18th month of life. Thereafter the mortality graph inclines steadily. The course of the graph is determined by very few diseases only, such as pituitary adenomas in both sexes, adenocarcinomas of the uterine glands in the females, and in the third and fourth year nephropathies in the males. Because of the high number of adenocarcinomas of the uterine glands the median life expectancy is between 30 and 33 months for the females, somewhat lower than between the 33rd and the 36th months for the males. The disease spectrum consists generally of tumor lesions, but out of the large tumor spectrum only single alterations exceed the 5% border. As shown by analysis of variance and estimation of the heritability coefficients (h2), mortality and the most important tumorous lesions underlie high genetic effects. The cage environments are found to have no influence upon mortality and diseases. Under the given standardized environmental conditions, the present results can be looked upon--because of the strong genetic effects on mortality and diseases - to be representative for correspondingly selected populations of future generations of the Han:Wistar stock.