Individual response to drugs is to a great extent influenced by hereditary factors. 2. The interaction between genetic and environmental factors affecting drug metabolism in man is of crucial importance to the understanding of therapeutic response in psychopharmacology. This proposition is illustrated for various psychotropic drugs such as the monoamineoxydase inhibitors (MAOI), tricyclic drugs and lithium salts as well as for enzymes involved in the synthesis and degradation of biogenic amines. 3. New perspectives in pharmacogenetics and their relevance to psychopharmacology are discussed.