Here we describe, for the first time, that budding yeast mitogen-activated protein kinase Hog1 and its upstream activators Pbs2 and Ssk1 are essential for the response to arsenite. Hog1 is rapidly phosphorylated in response to arsenite and triggers a transcriptional response that involves the upregulation of genes essential for arsenite detoxification.Saccharomyces cerevisiae sets up a battery of mechanisms in response to arsenic that involve transcription factors (Yap1 and Arr1/Acr1/Yap8) (20), cell membrane transporters (Arr3/ Acr3) (21), vacuole transporters that transfer arsenite-conjugated glutathione into the vacuole (Ycf1) (7), and an arsenate reductase (Arr2/Acr2) that reduces arsenate to arsenite before it can be a substrate of the transporters (15). However, it is still unknown if any of the mitogen-activated protein kinase (MAPK) pathways in Saccharomyces cerevisiae are involved in the response to arsenite.To study the function and importance of MAP kinase signaling in the response to arsenite in Saccharomyces cerevisiae, we monitored the sensitivity to arsenite of strains deficient in the MAPKs Hog1, Fus3, Kss1, and Slt2 as well as some of their upstream regulators, such as Ste7 and Bck1. As shown in Fig. 1, all the mutants and the wild type were able to grow at similar rates in rich medium (yeast extract-peptone-dextrose [YPD]). However, strains deficient in Hog1 (hog1⌬) did not grow in media containing sodium arsenite, indicating that Hog1 activity is necessary for the proper response of Saccharomyces cerevisiae to this metalloid.This sensitivity could be explained by the toxicity caused by sodium in hog1⌬ mutants (8). However, the presence of sodium in the media did not affect the growth of strains lacking Hog1 (Fig. 1), confirming that the toxicity of sodium arsenite in hog1⌬ mutants was produced by As(III).Some of the physiological effects caused by arsenite are thought to be produced by its capacity for producing reactive oxygen species (ROS). We monitored the sensitivity of the previously described mutants to hydrogen peroxide, a wellknown ROS producer. As shown in Fig. 1, all the mutants and the wild type showed similar sensitivity to hydrogen peroxide, indicating that arsenite produced its toxic effects in the cell through a mechanism that cannot be exclusively explained by its ROS production capacity.These results showed that Hog1 is required to respond to a