Periarteritis nodosa in spontaneously hypertensive rats -- incidence and distribution. Suzuki T,Oboshi S,Sato R Acta pathologica japonica SHR rats (Nihon Rat Inc.) were used to study spontaneous arterial lesions morphologically resembling human periarteritis nodosa (PN). Male SHR rats (159) developed PN in 76.1%, whereas females (39) in 10.3% during a period of 2 to 15 months of age. The frequency of PN in male SHR rats tended to increase with ageing and reached 100% at 9 months of age. PN lesions were widely distributed with only the lungs, brain and aorta being spared. PN in females was restricted to the tongue, parametrium and mesenterium. Male SHR rats with hypertension of more than 180 mm Hg showed PN in 81.7%, whereas in females with the same hypertension PN was found in only 17.4%. Although there was apparent sex difference in the incidence and background of hypertension, male SHR rats might be a useful animal model of human PN.