To analyze the biological pathways of the genetical activity in psychoses is growing more and more to an important goal. The research in this field should be started on the assumption of a multifactorial hereditary system controlling the somatic base of mental illness in a specific way as can be argued from twin and family studies. Screening the present data, there is strong evidence that the metabolism of affective psychoses is characterized by quantitative deviations only concerning the 5-hydroxytryptamine and norepinephrine turnover particularly. The biochemical findings in schizophrenic psychoses are suspicious for qualitative abnormities too. There are some indications of toxic products in the catecholamine metabolism and of antibodies against brain substances. All these disturbances could be caused by structural gene mutants or by mutations of the genetical regulatory system changing the sensitivity for the environmental stimulus. The simultaneous investigation of simple inherited serum groups is described as a useful tool for biological marking of the responsible genotypes.