Mechanism of interaction between biologically active substances and membranes as well as and membrane-bound enzymes of two types: mitochondrial monoamine oxidase (MAO) from rat liver tissue and Ca2+, Mg+2-dependent ATPase from sarcoplasmic reticulum (SR) were studied. All the substances studied inhibited most distinctly the SR ATPase as compared with mitochondrial MAO. EPR technique enabled to detect that BeSO4, aminazine, Pt- and Pd-5-sulpho-8-mercaptoquinolinate affected the membranes of both types decreasing the microviscosity in its hydrophobic part. K2PtCl4 and K2PdCl4 altered the conformation of membrane proteins. All the substances studied were bound by the same amino acid residues of proteins and affected similarly the structure lability of these membranes. As distinct from MAO, interaction of the substances with active sites of ATPase was responsible for pronounced differences in the inhibitory effect of the same substances on various membrane enzymes. Transformation of MAO, accompanied by a decrease in viscosity of hydrophobic part of mitochondrial membrane, contributed to appearance of new enzymatic properties, which appear to occur due to alteration in the protein conformation and to redistribution of the enzyme reactive sites situated relatively close to the membrane surface.