Acid phosphatase activity was studied histochemically in rat livers after perfusion fixation. The three different perfusates used were (1) 2% of paraformaldehyde; (2) 1.25% of glutaraldehyde, and (3) 1.25% of each of paraformaldehyde and glutaraldehyde. Unfixed fresh frozen sections showed high enzymatic activity in both centrilobular (CL) and periportal (PP) areas. A similar localization pattern resulted after perfusate 1 indicating a lack of reduction in the enzymatic activity in either of the two areas. After perfusate 2, PP arease remained intensely stained while slight reduction of the enzymatic activity was exhibited by the CL regions. Perfusion (3) resulted in higher activity in PP areas but maximal inhibition in the CL region. The inhibition of acid phosphatase activity in the CL areas correlates with regional differences of Kupffer cell population as well as the existence of metabolic heterogeneity between the two hepatic regions.