The objective of the present study was to clarify initial effects of aluminium (Al) on plasma-membrane integrity of soybean (Glycine max L. cv. Sito) root cells using net efflux of K+ and NO3- from isolated root tips (< 1.5 cm) as parameters. Increasing Al supply from 10-mu-M to 80-mu-M for up to 24 h severely depressed root elongation. In the root tips Al supply decreased the concentration of Ca considerably and of NO3- slightly whereas concentration of K+ was not affected. To determine net efflux, root tips were isolated after pretreatments with Al and incubated in distilled water (pH 4.1) for different time periods. The net efflux of both K+ and NO3- from isolated root tips was drastically depressed by Al supply, especially after incubation times of 1 and 3 h. The decrease in NO3- and K+ efflux was evident after only 2 h pretreatment with Al and became more distinct following Al treatments of 6 and 24 h. The results indicate that the structural integrity of plasma membranes of root cells is rapidly impaired by Al, leading to alteration of permeability.