1. The process for preventing tachyphylaxis of an indirect acting sympathomimetic amine drug selected from the group consisting of metaraminol, pseudoephedrine and methoxyphenamine which comprises administering to a patient concomitantly with said drug, a catecholamine precursor selected from the group consisting of tyrosine, a tyrosine precursor or a mixture of tyrosine and a tyrosine precursor to obtain a tyrosine blood plasma concentration between about 15 and about 100 .mu.g/ml.