A wild-type strain of the yeast Saccharomyces cerevisiae grown at a medium [Cu] of less than or equal to 50 nM contained less Cu,Zn superoxide dismutase (SOD) mRNA (60%), protein (50%), and activity (50%) in comparison with control cultures grown in normal synthetic dextrose medium ([Cu] approximately 150 nM). A compensating increase in the activity of MnSOD was observed, as well as a smaller increase in MnSOD mRNA. These medium [Cu]-dependent differences were observed in cultures under N2 as well. Addition of Cu2+ (100 microM) to Cu-depleted cultures resulted in a rapid (30 min) increase in Cu,ZnSOD mRNA (2.5-fold), protein (3.5-fold), and activity (4-fold). Ethidium bromide (200 micrograms/ml of culture) inhibited by 50% the increase in Cu,ZnSOD mRNA, while cycloheximide (100 micrograms/ml of culture) inhibited completely the increase in protein and activity. Addition of Cu2+ to greater than or equal to 100 microM caused no further increase in these parameters but did result in a loss of total cellular RNA and translatable RNA, a decline in the population of specific mRNAs, a decrease in total soluble protein and the activity of specific enzymes, and an inhibition of incorporation of [3H]uracil and [3H]leucine into trichloroacetic acid-insoluble material. Cu,ZnSOD mRNA, protein, and activity appeared relatively more resistant to these effects of Cu toxicity than did the other cellular constituents examined. When evaluated in cultures under N2, the cellular response to [Cu] of greater than or equal to 100 microM was limited to the inhibition of radiolabel incorporation into trichloroacetic acid-insoluble material. All other effects were absent in the absence of O2. The data indicated that medium (cellular) Cu alters the steady-state level of Cu, ZnSOD. This regulation may be at the level of transcription. In addition, Cu,ZnSOD exhibits the characteristics of Cu-stress protein in that it and its mRNA are enhanced relative to other cellular species under conditions of Cu excess. This observation and the O2-dependence of some of the manifestations of Cu excess suggest that one mechanism of Cu toxicity involves the superoxide radical anion O2-.