Histoohemioal and ultrastructural studies of aluminium-induced changes inrootmetabolism and the structure of the cytoplasmic organelles of the root cap indicated that the first observable effect of aluminium involved the migration of secretory vesicles of the Golgi apparatus which was considered indicative of interference by Al in membrane transport. Al also resulted in a reduction in the number of cisternae per dictyosome, disruption of the membranes of the cisternae and secretory vesicles as well as alteration to the appearance of the product elaborated in the secretory vesicles. Changes in the frequency and appearance of the endoplasmic reticulum were also noted. Progressive vacuolation of the cells of Al-treated roots was observed. No alteration to the nuclear structure of the cap initials involving either the appearance of the nuclear membranes, or degree of chromatin condensation could be connected to Al injury during the first 20 h of exposure. Al was shown to affect the pattern and intensity of respiratory activity in the root apex. High levels of respiratory activity were detected in control treatments in cells proximal to the cap/root junction and approximating the location of the quiescent centre. Al treatment was found to have a rapid inhibitory effect on the metabolic activity of these cells.S. Air. J. Plant Soil1985, 2: 8–17