No correlations were recognized between Al tolerance among four plant species, rice (Oryza sativa L.), maize (Zea mays L.), pea (Pisum sativum L.), and barley (Hordeum vulgare L.), in rank order of Al tolerance, and cation exchange capacities of root-tip (0-1 cm) cells or of their cell walls. The plasma membrane of root-tip of Al sensitive plant species (pea and barley) was considerably permeabilized with elongation of root in Al-free solution following 0.5 h pretreatment with Al. K+ release from and Al permeation into the protoplasts isolated from the root-tip of Al-sensitive plant species were more significant than those for Al-tolerant plant species (rice and maize) on 10 or 30 min treatment with Al. The permeability of the plasma membrane for protoplasts isolated from Al sensitive plant species was considerably increased by treatment with hy-potonic Al-free control solution following 10 min pretreatment with Al. To our knowlege, these are the most rapid responses to Al ions reported to date, i.e., within 0.5 h in whole plant and within 10 min in protoplast. These results suggest that a temporary contact with Al ions irreversibly alters the plasma membrane of root-tip cells of Al-sensitive plant species: the cells become more leaky and rigid due to binding of Al ions to the plasma membrane.