This study investigated the effects of dietary tyrosine or tryptophan supplementation on immunity and brain neurotransmitter levels after antigen challenge. Chickens were given 0.1%, 0.5% or 1% supplemental dietary L-tyrosine or L-tryptophan prior to the injection of sheep red blood cells (SRBC). The 0.1% tyrosine supplementation increased primary IgM and secondary IgG titers at some time periods in Leghorns and decreased secondary IgM titers at Day 5 in broilers, while 0.1% tryptophan addition decreased Leghorn secondary IgM titers and increased secondary IgG titers at Day 9 and broiler secondary IgM titers at Day 9. The phytohemagglutinin (PHA) wattle response in Leghorns and broilers and resistance of Leghorns to E. coli challenge were not affected with the 0.1% supplemental level. With higher levels, 0.5% tyrosine supplementation increased Leghorn primary IgM titers at Day 11. In broilers, 0.5 and 1% tryptophan supplementation decreased secondary total antibody titers at Day 2, while the secondary IgM titers at Day 6 with the 0.5% tyrosine supplementation were higher than those with 1% tyrosine or tryptophan supplementation. The 0.5% or 1% tryptophan supplementation also lowered stressed broiler primary IgM titers at Day 3. After pooling the titer results within two dietary supplemental levels (0.5% and 1%) of a given amino acid, the tyrosine treatment appeared to suppress antibody response in unstressed broilers, but not stressed ones, while tryptophan displayed a suppressive trend in broilers under both situations. Supplementation with 0.5% or 1% dietary tyrosine did not alter brain catecholamine or serotonin (5-HT) levels in Leghorns. In contrast, 0.5% or 1% dietary tryptophan supplementation generally increased 5-HT, and its metabolite 5-HIIAA, and S-HIAA/5-HT ratios in the diencephalon, telecephalon, and brain stem in Leghorns. Tryptophan supplementation caused a dose-response increase in S-IIT and S-IHIAA levels of the brain stem. The results demonstrated that 0.5-1% dietary tryptophan supplementation suppressed broiler, but not Leghorn, antibody response, and the mechanism was probably via enhancing 5-IIT synthesis and release. The results also suggested that lower levels (0.1-0.5%) of dietary tyrosine supplementation may enhance immune response in Leghorns and stressed broilers, but ingesting large quantity of tyrosine, occurred in broilers, suppresses the response. / Master of Science