Saccharomyces cerevisiae can utilize allantoin as a sole nitrogen source by degrading it to ammonia, "CO(2)," and glyoxylate. We have previously shown that synthesis of the allantoin degradative enzymes is contingent upon the presence of allophanate, the last intermediate in the pathway. The reported repression of arginase by ammonia prompted us to ascertain whether or not the allantoin degradative system would respond in a similar manner. We observed that the differential rates of allantoinase and allophanate hydrolase synthesis were not decreased appreciably when comparing cultures grown on urea to those grown on urea plus ammonia. These experiments were also performed using the strain and conditions previously reported by Dubois, Grenson, and Wiame. We found allophanate hydrolase production to be twofold repressed by ammonia when that strain was grown on glucose-urea plus ammonia medium. If, however, serine or a number of other readily metabolized amino acids were provided in place of ammonia, production of the allantoin degradative enzymes was quickly (within 20 min) and severely repressed in both strains. We conclude that repression previously attributed to ammonia may result from its metabolism to amino acids and other metabolites.