Inhibitors of energy metabolism (3-ohlorophenylhydrazonomalononitrile, antimycin A, iodoacetamide, dicyclohexylcarbodiimide) but not of transport (uranyl ions) stimulate at low concentrations the uptake ofl-leucine,l-glutamic acid,l-argimne and, to a lesser degree, of 2-aminoisobutyric acid inSaccharomyces cerevisiae. The effect is apparent only after augmenting the energy reserves of cells by preincubation withd-glueose or, more strikingly, with ethanol. It is absent in a mutant (op1) lacking the translocation system for ADP-ATP in mitochondria. The presence of two different energy reserves for amino acid transport is indicated (one in energy-poor, the other in energy-rich cells). The stimulating effect appears to be caused by a retarded degradation of the transport proteins as occurs at a lowered level of mitochondria-produced ATP.