Na(+)/H(+) exchange activity in barley (Hordeum vulgare cv CM-72) root tonoplast was induced by Na(+) even in the presence of inhibitors of protein synthesis. Induction occurred with a half-time of only 15 minutes. When salt-treated roots were transferred to a nutrient solution containing no Na(+), the activity disappeared with a similar time course. The data suggest that Na(+)/H(+) exchange was due to activation of an existing protein rather than to de novo protein synthesis.