We investigated the cause and effect relationships among ethylene, polyamines, and K+ in barley (Hordeum vulgare L. cv. Amagi) seedlings, Application of 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor of ethylene, to the growth medium caused a decrease in K+ concentration in roots and an increase in shoots. Addition of ACC induced putrescine accumulation in roots, while spermidine and spermine levels remained unchanged. Exogenous supply of putrescine led to putrescine accumulation and reduced K+ concentration. Application of Co2+, an inhibitor of ethylene biosynthesis, together with ACC, inhibited putrescine accumulation with a decrease in K+ concentration in roots, ACC-treated roots showed K+ uptake capacity equivalent to that of control roots, implying that the majority of K+ is translocated to shoots. These results suggest that ethylene regulates K+ partitioning between roots and shoots through the level of accumulation of putrescine in barley seedlings.