METYROSINE INHIBITS TYROSINE HYDROXYLASE, WHICH CATALYZES THE FIRST TRANSFORMATION IN CATECHOLAMINE BIOSYNTHESIS, I.E., THE CONVERSION OF TYROSINE TO DIHYDROXYPHENYLALANINE (DOPA). BECAUSE THE FIRST STEP IS ALSO THE RATE-LIMITING STEP, BLOCKADE OF TYROSINE HYDROXYLASE ACTIVITY RESULTS IN DECREASED ENDOGENOUS LEVELS OF CATECHOLAMINES, USUALLY MEASURED AS DECREASED URINARY EXCRETION OF CATECHOLAMINES AND THEIR METABOLITES. THE MAXIMUM BIOCHEMICAL EFFECT USUALLY OCCURS WITHIN TWO TO THREE DAYS, BLOOD PRESSURE DECREASES PROGRESSIVELY DURING THE FIRST TWO DAYS OF THERAPY WITH METYROSINE; AFTER WITHDRAWAL, BLOOD PRESSURE USUALLY INCREASES GRADUALLY TO PRETREATMENT VALUES WITHIN TWO TO THREE DAYS.