The kinetics of norepinephrine (NE) metabolism in the heart was studied in genetically hypertensive and normotensive control Wistar rats. Concentrations of endogenous NE were similar in the two groups. However, rates of synthesis of NE were reduced in these hypertensive rats, whether calculated from the rate of decline (fractional turnover rate) of cardiac tritiated NE (3H-NE) after intravenous injection, or estimated from levels of14C-NE in the heart after injection of the precursor14C-L-tyrosine. In experiments with3H-NE the synthesis rate of NE was 30.2 ng/hour/g heart in control and 18.2 ng/hour/g heart in hypertensive rats. The levels of14C-NE found in the heart of normotensive rats given14C-tyrosine were up to 1.4 times those found in hypertensive rats. These findings indicate a reduced rate of release of NE in this form of hypertension and, rather than implicating NE as a primary factor, suggest a secondary, compensatory mechanism.