In the yeast Saccharomyces cerevisiae, the expression of resistance to the L-arginine analog, L-canavanine, after mutagenesis, is strongly dependent on the metabolic state of the cell. The frequency of mutations recovered after exposure to ultraviolet light or X rays was measured under a variety of culture conditions. The results indicate that the frequency of mutants recovered is determined by the following three factors: (1) The potential mutants still possess enough permease activity to take up some of the cell poison, and some are therefore killed before they can express the mutant genotype. The sensitivity is strongly influenced by the endogenous free arginine, which is in turn influenced by the growth medium. (2) The rapid decay of the permease molecules and the inability of the potential mutants to resynthesize this protein results in a rapidly increasing change of expression when selection is delayed. (3) During the time when the permease activity is decaying, repair of the mutagen-induced damage appears to occur.