Repeated injections of sufficient doses of tyramine (20 mg/kg i.p.) to rats at 15-min intervals for a total of 12 doses, deplete the heart norepinephrine (NE) stores by about 90 %. Following the last dose of tyramine, the concentration of heart NE increased progressively as the net result of the processes of synthesis and loss. The net rate of NE repletion is given by the equation: log[(([NE]∞ – [NE]) [NE]∞)/[NE]∞] –kt/2.303 The calculated ‘k’ constant times the NE steady-state level corresponds to the NE synthesis rate ‘K’. This rate is identical with that determined from the decline in levels after α-methyltyrosine (α-MT) and from the loss of 3H-NE after labeling with tracer doses.