Abstract: The relationship between lead treatment and changes in the concentration of serum, liver and brain high and low molecular weight alkaline phosphatase has been investigated in this manuscript. Results obtained showed that every other day intrapritoneally injection of 39.5 μmol kg-1 of lead as (Pb(CH3COO)2.3H2O), in male rats for 2 consecutive weeks resulted in decreasing level of liver and brain alkaline phosphatase by 16.7 and 10.9%, respectively, whereas an elevation of serum enzyme activity by 28.4% was seen in comparison to untreated controls (p<0.05). Long-term exposure to 13.2 μmol kg-1 of this salt, showed a statistically significant reduction in liver and brain levels of alkaline phosphatase by 18.7 and 13.2%, respectively and an increment in serum activity of the enzyme by 37.6% in compared to control group (p<0.05). Using gel filtration chromatography technique with sephacryl S300 showed that, in comparison to control groups, serum and liver homogenate from lead treated groups had a significant level of high molecular weight alkaline phosphatase, which might be considered as a potential biomarker for lead toxicity.