The present study demonstrated a significant variation of liver aldehyde oxidase activity in twelve strains of rats. The highest activity was found with Sea:SD rats and the lowest one with WKA/Sea rats. When assayed using benzaldehyde as a substrate, the difference in the activity between the two strains was 63.5-fold. Little significant differences in Km values of the enzyme were observed among several strains. On the other hand, Western blot analysis for the enzyme suggested that the strains possess different quantities of the enzyme, which are roughly parallel to aldehyde oxidase activity observed in the strains.