Women with untreated phenylketonuria (PKU), tend to give birth to infants with multiple congenital anomalies, as elevated maternal phenylalanine (Phe) level is teratogenic. The best outcomes occur when strict control of maternal Phe levels is achieved before conception and maintained throughout pregnancy. Such diets are not highly palatable and therefore there is often loss of dietary compliance. An alternative to low Phe diet would be a more normal diet that is altered so that the Phe content is less problematic. Hence, there is an interest in the use of large neutral amino acids (LNAA), which compete with Phe for membrane transport sites in the intestines and the placental barrier and thus alleviate the problems associated with PKU. In our research we used a PKU mouse model to examine the effect of LNAA supplementation on the maternal and fetal blood Phe levels. 3 different doses of LNAA supplementation were given to different animal groups to observe its effect on the blood Phe levels. Results showed that 16.7% and 33.4% LNAA supplementation caused significant decrease in the blood Phe levels over a period of time, but was associated with adverse effects.