The influences of paraffin and GMA-embedding on acid phosphatase, esterase and beta-glucuronidase activity of differently fixed or freeze substituted rat livers were studied. 1. Embedding generally causes a reduction of the enzyme activities but improves considerably the quality of the microscopical pictures when compared with appropriate cryostat sections. Embedding therefore may serve as a very useful tool for detail studies on the cytological level. 2. The embedding media act differently on the reactive sites: a. Paraffin causes a heavy denaturation of the enzyme activity in lysosomes but preserves the activities of the ergastoplasmic (= "microsomal") enzymes. The degree of denaturation increases with increasing embedding temperature. b. GMA-embedding delivered opposite effects by preserving lysosomal activities and quenching endoplasmic enzymes. UV-polymerization of GMA causes a general inactivation of enzymes. 3. The histochemical reactivity of substrates such as glycogen was not influenced by the embedding. However, its most natural localization is achieved by freeze drying or isopropanol freeze substitution followed by GMA-embedding.