Adrenal vein catheterizations were done in SHR and control rats at different ages during the development of hypertension. All SHR became hypertensive by 15 weeks of age. The secretion rate of aldosterone was significantly reduced at 8 weeks of age, 18-hydroxy-11-deoxycorticosterone (18-OH-DOC) at 12 weeks of age, deoxycorticosterone (DOC) at twenty weeks of age, and corticosterone (B) at 12 and 20 weeks of age. Secretion data suggest either an enzyme block, or increased conversion of known steroids to an unknown steroid product. Reduced secretion of corticosterone could explain the adrenal hyperplasia observed in SHR which may be important to the development of hypertension in these animals.