Abstract: Alkaline phosphatase is a family of ecto-phosphomonoesterases that can be resolved into two bands on polyacrylamide gel electrophoresis, a high-molecular mass form (Mr>1 000 000) and a low molecular mass form (Mr: 150000). The relationship between aluminium treatment and changes in the activity of serum, liver and brain high and low molecular weight alkaline phosphatase has been investigated in this manuscript. Results obtained from in vivo study showed that every other day intrapritoneally injection of 186 μmol kg-1 of aluminium (AlCl3.6H2O), in male rats for 2 weeks resulted in decreasing the level of liver and brain alkaline phosphatase by 14.9 and 9.9%, respectively, whereas an elevation of serum levels of this enzyme by 21.1% was seen in comparison to untreated controls (p<0.05). Long-term exposure of 74.5 μmol kg-1 of this salt, showed a statistically significant reduction in liver and brain level of alkaline phosphatase by 15.8 and 12.3%, respectively and an increment in serum activity of the enzyme by 30.9% in compared to control group (p<0.05). Gel filtration chromatography technique with sephacryl S300 showed that, in comparison to control groups, serum and liver homogenates from aluminium treated groups had a significant level of high molecular weight alkaline phosphatase. In vitro experiments showed that aluminium inhibited all the isoenzymes non-competitively. Low molecular weight alkaline phosphatases were more heat and urea stable than high molecular weight fractions.