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IN A previous paper (Shao and Hill, 1968) it was reported that thiouracil as a dietary supplement reduced the concentration of free lysine, threonine, tryptophan, methionine and leucine in the blood plasma of chicks. The present paper gives the results of experiments in which thiouracil and/or iodinated casein were added to a basal diet and extends the observations to several additional amino acids. EXPERIMENTAL The basal diet, essentially a wheat, corn soybean meal mixture, was described in detail by Shao and Hill (1968). White Plymouth Rock pullets were fed this diet from day old to 7 days of age and then divided on the basis of weight into lots of 12 chicks each. The experimental diets were assigned to these lots at random, each diet to 4 lots and the experiments continued for another 14 to 15 days. Blood samples were removed from each chick in two lots per treatment…

LEVELS of free amino acids as determined in the blood plasma of chicks can be quite variable even though the chicks are fed similar diets and are of the same breed, sex and age (Shao and Hill, 1967; Hill and Olsen, 1963). An unusually high level of lysine and threonine observed in an experiment in which blood was sampled under conditions of extremely high summer temperature (Hill and Olsen, 1965), prompted us to consider the possibility that the metabolic rate of the bird at the time of sampling might influence plasma amino acid levels. As a test of this hypothesis we measured the plasma amino acids following administration of thiouracil at a level which might be presumed to lower metabolic rate (Singh and Shaffner, 1950; McCartney and Shaffner, 1950, 1949; Shaffner and Andrews, 1948). Although the observations in the present paper did not support the above hypothesis they are believed . . .