The aim of this study was to understand the effects of the interaction between aluminium and low pH in a native fish species Prochilodus lineatus. Thus, juveniles of this neotropical fish species were exposed to 196 microg L(-1) of dissolved aluminium in acid water (Al group), only to acid water (pH group) or to water with neutral pH (CTR group) for 6, 24 and 96 h. Al effects were evaluated with regard to hematological parameters (hemoglobin, hematocrit and red blood cell number), plasma ions and osmolarity, density and distribution of chloride cells (CC), Na(+)/K(+)-ATPase activity in the gills, metabolic (protein and glucose) and endocrine (cortisol) parameters. The fish exposed to Al had increased hematological and metabolic parameters in relation to the CTR group after all periods of exposure. In fish exposed to Al for 24 and 96 h plasma ions and osmolarity were significantly lower and the identification of the enzyme Na(+)/K(+)-ATPase by immunohistochemistry indicated a reduction in the number of CC in the gills. Enzyme activity was 50% lower in fish exposed to Al in all experimental times. Taken together these results showed that acute exposure to Al causes an ionic unbalance, probably related to the effects of Al on Na(+)/K(+)-ATPase activity, on the distribution and number of chloride cells in the gills as well as the effects associated with the stress response caused by the presence of the metal.