Estimations of prekallikrein and kallikrein inhibitor by means of TAME-esterase method and of the content of kininogen by biological procedure were carried out in blood plasma, isolated from right and left ventricles of heart, of rats with spontaneous hypertension, Okamoto-Aoki strain, and of the normotensive animals Wistar-Kyoto strain. The animals were 2=3 months, 6=8 months and 12=14 months old. Development of spontaneous hypertension was accompanied by an increase in the kallikrein-kinin system components studied. Content of prekallikrein and kininogen was higher in blood plasma from left heart ventricle than in blood from right ventricle, independently of the age of the rats, by 6=10% and 33=40%, respectively. Lung tissue was shown to carry out a definite reserve function in the compensatory increase of the kallikrein-kinin system potential under conditions of spontaneous hypertension.