Neurospora crassa can utilize various purine bases such as xanthine or uric acid and their catabolic products as a nitrogen source. Four classes of mutants which affect the purine degradative pathway were isolated and studied. Mutants of the aln-1 class specifically lack allantoinase, while alc-1 mutants lack allantoicase. Mutants designated as xdh-1 cannot utilize hypoxanthine as a nitrogen source and are presumed to be deficient in xanthine dehydrogenase activity. A regulatory mutant, amr, was found to have only very low, uninduced levels of uricase, allantoinase, and allantoicase. None of these genes are closely linked to each other. The three initial enzymes involved in the catabolism of uric acid are controlled in a complex manner by both induction and repression. Several lines of evidence indicate that the true inducer of uricase and allantoicase is uric acid. The use of the newly isolated mutant strains made it possible to demonstrate that neither allantoin nor allantoic acid could act as inducers. Furthermore, hypoxanthine itself was shown to be ineffective as an inducer although it can be metabolized to form an inducer. A non-metabolizable analogue of uric acid, 8-azaxanthine, is a gratuitous inducer of these enzymes. Uricase and allantoicase were found to be synthesized coordinately, but they were not coordinately regulated with allantoinase. Both uricase and allantoicase are stable enzymes and do not undergo turnover; nor are they subject to feedback inhibition by ammonia. Allantoinase, however, is quite labile both in vivo and in vitro. This enzyme was found to turnover in vivo in the presence of cycloheximide with a half-life of approximately 20 minutes. The amr (for ammonia regulation) mutant cannot utilize a wide range of compounds, including purines, nitrate, and many amino acids as a nitrogen source and also displays a multiple enzyme loss. The amr gene appears to play a major role in the control of nitrogen metabolism. It is postulated that the amr locus encodes a regulatory protein which is required to activate transcription of the structural genes for a group of related enzymes involved in nitrogen metabolism.