The osmotic balance between the cytoplasmic and extracellular compartments of cells is critical for the control of cell volume. A mammalian protein kinase, Jnk, which is a distant relative of the mitogen-activated protein kinase group, was activated by phosphorylation on threonine and tyrosine in osmotically shocked cells. The activation of Jnk may be relevant to the biological response to osmotic shock because the expression of human Jnk in the yeast Saccharomyces cerevisiae rescued a defect in growth on hyper-osmolar media. These data indicate that related protein kinases may mediate osmosensing signal transduction in yeast and mammalian cells.