Repeatedly bred male and female rats of many strains develop hyperglycemia, hyperlipidemia, hypertension, and arteriosclerosis spontaneously. The intensity of their arterial disease and related metabolic derangements appear to be related to their reproductive activity. Repeatedly bred spontaneously hypertensive rats (SHR) were found to have severe hypertension, hyperglycemia, hyperlipidemia, elevated creatine phosphokinase (CPK), serum glutamic oxaloacetic and glutamic pyruvic transaminase (SGOT, SGPT), and lactic dehydrogenase (LDH), as well as high circulating corticosterone levels. Despite these atherogenic metabolic derangements and their severe hypertension, the breeder SHR did not develop the severe, generalized arteriosclerosis found in other strains of breeder rats. Instead, the arterial lesions, consisting of intimal hyalinization and fibrosis, medial hypertrophy, and occlusion of the lumen, were found only in male breeder SHR and were confined to the intratubular arteries of the testes. It is suggested that the severe hypertension, genetic influences, or differences in hypothalamic-pituitary-adrenal-gonadal function in breeder SHR may not have been conducive to the development of arteriosclerosis in this particular strain of rats.