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Experiments have been conducted to determine the influence of the non-essential amino acids on the amounts of methionine, isoleucine and threonine required to maintain adult male rats in nitrogen equilibrium. Cystine was the only non-essential amino acid which influenced the amount of methionine required for this purpose. In the absence of cystine, an intake of 14.6 mg DL-methionine was required; when the animals received 10 mg of L-cystine, 4.2 mg DL-methionine were sufficient; and when the cystine ingested was increased to 21 mg per day, 3.2 mg DL-methionine were needed for maintenance of nitrogen equilibrium. The isoleucine and threonine requirements, 11 and 10 mg per day, respectively, were unaffected by the non-essential amino acids at the levels of intake used.