Two distinct regulatory mechanisms are responsible for the absence of general amino-acid permease activity in cells of the wild-type strain sigma 1278b of Saccharomyces cerevisiae grown in the presence of ammonium ions. One is a reversible inactivation process which progressively develops upon addition of ammonium ions to a proline-grown culture, and completely suppresses the permease activity within one hour. This inactivation process is absent in mutants altered at the MUT2, MUT4, or PGR genetic loci. In these mutants, a repression of the formation of active permease may clearly be observed in the presence of ammonium ions. This second regulatory mechanism is absent in mutants affected at the GDHCR locus, which might code for a repressor molecule. It is also relieved in the presence of a glnts mutation (which makes the glutamine synthetase thermosensitive) suggesting glutamine as an effector. Two other ammonia-sensitive permeases, namely the proline permease and the ureidosuccinic-acid permease, seem to be subject to the same double regulation. Mutations affecting the structural gene of the anabolic NADP-linked glutamate dehydrogenase (gdhA) seem to completely prevent repression of the general amino-acid permease, while they partially suppress its inactivation in the presence of ammonium ions.