Free oxygen radicals are an irrefutable component of life, underlying important biochemical and physiological phenomena in animals. Here it is shown that free oxygen radicals activate plasma membrane Ca(2+)- and K(+)-permeable conductances in Arabidopsis root cell protoplasts, mediating Ca(2+) influx and K(+) efflux, respectively. Free oxygen radicals generate increases in cytosolic Ca(2+) mediated by a novel population of nonselective cation channels that differ in selectivity and pharmacology from those involved in toxic Na(+) influx. Analysis of the free oxygen radical-activated K(+) conductance showed its similarity to the Arabidopsis root K(+) outward rectifier. Significantly larger channel activation was found in cells responsible for perceiving environmental signals and undergoing elongation. Quenching root free oxygen radicals inhibited root elongation, confirming the role of radical-activated Ca(2+) influx in cell growth. Net free oxygen radical-stimulated Ca(2+) influx and K(+) efflux were observed in root cells of monocots, dicots, C3 and C4 plants, suggesting conserved mechanisms and functions. In conclusion, two functions for free oxygen radical cation channel activation are proposed: initialization/amplification of stress signals and control of cell elongation in root growth.