Six human males each received 0.56 g phenylalanine (Phe) in the form of 1.0 g aspartame or 12.2 g bovine albumin in 200 ml water or water alone. Venous blood samples collected before consumption and during the following 4 hr were assayed for plasma levels of large, neutral amino acids (LNAA), aspartate, insulin and glucose. The area under the curve for plasma Phe was 40% greater, although not significant, after aspartame compared with albumin intake. The indicated increased clearance rate of plasma Phe after albumin may be caused by the significant increase of insulin, on which aspartame had no effect. There was a significant main effect of aspartame for plasma tyrosine but not for tryptophan, valine, isoleucine or leucine. Plasma aspartate was significantly increased at 0.25 hr after the aspartame intake. The percentage Phe/LNAA decreased slightly in response to albumin but increased 55% after aspartame and remained significantly increased for 2 hr. Tyrosine/LNAA increased and tryptophan/LNAA decreased modestly after aspartame intake. The study showed that the intake of aspartame in a not unrealistically high dose produced a marked and persistent increase of the availability of Phe to the brain, which was not observed after protein intake. The study indicated, furthermore, that Phe was cleared faster from the plasma after consumption of protein compared with aspartame.