The consequences of dietary excesses of 10 essential amino acids, His, Ile, Phe, Trp, Val, Arg, Leu, Lys, Met, Thr, on growth, feed intake and plasma levels of triiodothyronine (T3) and thyroxine (T4) in growing chicks were investigated. Each amino acid was added to a starter ration to bring it to a level 2.84x above the National Research Council (1984) requirement. Excesses of all amino acids except His and Leu caused significant reductions in weight gain. Of the amino acid excesses that reduced growth, only Trp and Val did not also reduce feed intake. Gain:feed decreased significantly only in chicks consuming excess Arg, Lys, Phe, and Trp. Chicks fed excesses of Ile and Val had plasma T3 levels that were statistically higher than control levels; none of the other amino acid excesses significantly altered blood concentrations of this hormone. Compared to the control, plasma T4 levels were not significantly altered by the amino acid excesses, but there was a significant difference between Trp and Val, the latter being lower. This study shows that high dietary levels of essential amino acids cause depressions in weight gain and feed intake, and, with Ile and Val, these depressions are accompanied by elevations in plasma T3 levels. Otherwise, the amino acid excesses had little effect on plasma levels of thyroid hormones.