Toxicological investigations are vital in the development of drugs and chemicals, in which enzymatic catalysis plays a key role for the visualization of the binding sites. It is I.-or example used in techniques like immunohistochemistry or in situ hybridization (ISH), obtaining information on the protein and genetic level. In the immunohistochemistry tissue constituents are identified with a specific labeled antibody. During ISH specific nucleic acid RNA of DNA sequences are identified by complementary base pairing with labeled probe strands of RNA, DNA or oligonucleotides. For the latter technique, the procedure, probe molecules, possible enzymes and labeling methods and the sensitivity are discussed. Finally, the use of enzymatic catalysis in toxicologic pathology, for example, in carcinogenicity investigations or in oncology, is discussed.