Growth of Saccharomyces cerevisiae on poor nitrogen sources such as allantoin or proline was totally inhibited by addition of a non-degradable basic amino acid to the medium. Cells treated with lysine contained greatly reduced quantities of histidine and arginine. Conversely, lysine and histidine were severely reduced in arginase-deficient cells treated with arginine. When all three basic amino acids were present in the culture medium, growth was normal suggesting that synthesis of all three basic amino acids was decreased by an excess of any one of them. Inhibition of growth was accompanied by a fivefold increase in the observed ratio of budded to unbudded cells. These morphological changes suggested that DNA synthesis was inhibited. Consistent with this suggestion, addition of a basic amino acid to the culture medium substantially reduced the ability of the cells to incorporate [14C]uracil into alkali-resistant, trichloroacetic acid-precipitable material. RNA and protein synthesis, although decreased, were less sensitive to the effects of such additions.