Mutants lacking the general amino acid permease activity fall into two classes of complementation. Mutations at the GAP1 locus abolish the general amino acid permease activity specifically, while those in the NPR1 locus simultaneously affect several other ammonia-sensitive uptake systems. The NPR1 locus as well as the GAP1 locus code for proteins, as shown by the identification of nonsense mutations in both these genes. Frameshift mutations in the GAP1 locus, and conditional, thermosensitive, mutations in the NPR1 locus were also obtained. No intragenic complementation was detected among 33000 crosses between gap1- mutant strains. Mutations at three unlinked loci, namely MUT2, MUT4, and PGR, release one of the two controls which prevent expression of the GAP1 gene in ammonia-grown yeast cells. The pgr regulatory mutation is located in the GAP1 locus near one end of this region, the fine structure of which has been determined by X-ray-induced mitotic recombination. On the basis of the properties of the mutants it is likely that the PGR region determines a receptor site for the negative control mediated by the products of the MUT2 and MUT4 genes. The data presented here are compatible with this negative control operating either at the transcriptional or at a post-transcriptional level of the GAP1 gene expression. The present work initiates the study of the regulation of the general amino acid permease at the molecular level.