Genetic factors involved in spontaneous hypertension were analyzed in the F₂ generation obtained between spontaneously hypertensive and normotensive rats. Positive correlations to blood pressure were observed in the heart, pituitary and adrenal weights, and in the thyroidal acid phosphatase activity. A weak inverse correlation was observed between blood pressure and aroma-tic L-amino acid decarboxylase activity of the brainstem. The F₂ with the hypertensive pattern of renal esterase isozymes showed a significantly higher blood pressure than the F₂ with the normotensive pattern. These findings correlated to blood pressure may be related to the genetic pathogenesis of spontaneous hypertension except for the secondary effect of hypertension.