Acid phosphatase activity and its sensitivity to formalin inhibition were compared histochemically in cryostat sections of human BPH and rat ventral prostate. While the glandular epithelium in both human and young adult rat ventral prostate exhibits positive acid phosphatase activity, the intensity of the reaction is greater in human tissue. Moreover, human prostatic tissue remains acid phosphatase positive following 24 hours immersion in formalin, whereas the enzyme reaction in young adult rat ventral prostate disappears after 5 hours of formalin treatment. Unlike both human and young adult rat ventral prostate, sites of acid phosphatase activity in aged rat ventral prostate are not homogeneously distributed throughout the glandular epithelium but, rather, appear as large clumps, which persist after 5 hours of formalin treatment. Procedures used for the histochemical demonstration of formalin-resistant human prostatic acid phosphatase are not, therefore, directly applicable to rat ventral prostate.