Abstract 1. 1. Tryptophan is capable of inducing a number of enzymes. The tyrosine aminotransferase activity will increase about 6 hr after an injection of 300 mg/kg dl-tryptophan. 2. 2. l-methionine (300 mg/kg) was found to cause a relatively small increase of the tyrosine aminotransferase activity already after 1 hr. 3. 3. A considerable increase of the activity of tyrosine aminotransferase could be obtained when dl-tryptophan was combined with l-methionine or cortisone acetate. 4. 4. To study the mechanism of tryptophan action, 6-aminonicotinamide and nicotinic acid were injected. 6-Aminonicotinamide inhibited the effect of tryptophan if administered together, while nicotinic acid increased just slightly the activity of tyrosine aminotransferase.