Sexually mature, male and female Sprague-Dawley rats were housed in large communal breeding cages or in smaller paired breeding cages. Virgin control rats of the same age were housed similarly but segregated by sex. Breeders became obese, developed a fatty liver, and showed elevated levels of triglycerides, free fatty acids, and cholesterol. Breeders had high blood pressure, enlarged hearts, hyperglycemia, and islet beta cell degranulation. Serum enzymes, creatine phosphokinase, serum glutamic oxalo-pyruvic transaminase, serum glutamic pyruvic transaminase, lactate dehydrogenase, and blood urea nitrogen levels were elevated in breeder rats. The adrenal glands of male breeders appeared hyperactive; the adrenal glands of female breeders were thrombosed and appeared to be hypoactive. Male breeder rats developed microscopic aortic lesions only; female breeders developed advanced calcific aortic sclerosis. Male breeders kept in active stud service manifested the most abnormal metabolic and pathophysiological changes. Female breeders developed similar pathophysiological changes after four pregnancies, irrespective of their paired or communal breeding environment. Virgin rats were normal regardless of housing conditions. Our findings suggest that repeated breeding in male and female rats causes resetting of the hypothalamic-pituitary-adrenal-gonadal axis. This may lead to disturbed hormonal and metabolic changes which culminate with the development of accelerated cardiovascular degenerative changes.