The mechanism(s) and site(s) of action of volatile inhaled anesthetics are unknown in spite of the clinical use of these agents for more than 150 years. In the present study, the model eukaryote Saccharomyces cerevisiae was used to investigate the action of anesthetic agents because of its powerful molecular genetics. It was found that growth of yeast cells is inhibited by the five common volatile anesthetics tested (isoflurane, halothane, enflurane, sevoflurane, and methoxyflurane). Growth inhibition by the agents is relatively rapid and reversible. The potency of these compounds as yeast growth inhibitors directly correlates with their lipophilicity as is predicted by the Meyer-Overton relationship, which directly correlates anesthetic potency of agents and their lipophilicity. The effects of isoflurane on yeast cells were characterized in the most detail. Yeast cells survive at least 48 h in a concentration of isoflurane that inhibits colony formation. Mutants resistant to the growth-inhibitory effects of isoflurane are readily selected. The gene identified by one of these mutations, zzz4-1, has been cloned and characterized. The predicted ZZZ4 gene product has extensive homology to phospholipase A2-activating protein, a GO effector protein of mice. Both zzz4-1 and a deletion of ZZZ4 confer resistance to all five of the agents tested, suggesting that signal transduction may be involved in the response of these cells to volatile anesthetics.